Part One: Height Advice.
When I first read the question title, I wondered what difference there would be between the size of a teenage boy and the size of a full grown man. When I read the boy Ryker described as being "Sixteen years old. About five feet tall." I thought that was very improbable.
Other people have also commented that Ryker's age and size seem improbable to them.
I later remembered the great differences in the growth rates of various children and that the growth rates which were familiar and normal to me were not the only possible growth rates for boys.
If Alendyias is from a time and place where it was normal and common for boys to be "Sixteen years old. About five feet tall." and so thinks that is the normal size for boys that age everywhere and every time, maybe they should set their story in that time and place, or select another time and place when and where it was common for sixteen year old boys to be about five feet tall.
And before Ryker has his magical instant growth spurt, Alendyias should show the readers that most sixteen year old boys there are only about five feet tall. Possibly a boy at school should be described as being almost six feet tall and a giant standng head and shoulders above the other sixteen year boys in the class. Perhaps Ryker's height will be measured some time befor he grows.
And Alendyias may need to state that it was normal then and there for Ryker and other sixteen year old boys to be that small.
In most societies there is a considerable spread in the rates of growth of boys and the heights of boys at the same age. And there is also a considerable spread in the rates of growth of boys between different societies at the same era, or between different eras of the same society.
Robert Pershing Wadlow (1918-1940) was the tallest medically record human, reaching a height of eight feet 11.1 inches. At the age of 16, he was eight feet 1 1/4 inches tall. https://en.wikipedia.org/wiki/Robert_Wadlow
The shortest medically recorded man was Chandra bahadur Dangi (1939-2015) of Nepal, at 21.5 inches or 1 foot 9.5 inches. https://en.wikipedia.org/wiki/Chandra_Bahadur_Dangi I find it easy to believe he might have been a couple inches taller at age 16 then when his age was measured as an old man, but I doubt whether he was ever much more than two feet tall.
Even among normal sized people without any medical conditions affecting their growth, probably at least one percent of normal growing boys will be at least ten percent taller or shorter than most boys their ages in their society.
Part Two: Some personal examples.
I remember that when I went to high school most of the boys in my class grew to close to their final adult heights, maybe five feet eight or five feet ten inches tall, when aged fifteen or sixteen. I guess I reached about five feet nine or ten inches when I was fifteen, and considered myself to be almost six feet tall.
I remember when I saw Shenandoah (1965) an early scene had the Anderson family at the table. The imaginatively named "Boy" Anderson was asked how old he was and said eighteen, but his father said he was sixteen. And since I think he was about five tall I thought he looked more like a twelve year old boy to me.
Actor Philip Alford was born September 11, 1948 according to IMDB https://www.imdb.com/name/nm0019221/?ref_=nv_sr_srsg_0 And the filming period for the movie included 11 August 1964 https://www.imdb.com/title/tt0059711/locations?ref_=ttrel_sa_4 when pHilip Alford was 15 years and 11 months old.
There were thirty four boys in my small class when we graduated. A few of them were shorter than most. In one of the pictures for the yearbook in the year we graduated, when we should have all been eighteen, the smallest boy in my class is standing next to me. I once measured that photo and found he was a little more than 0.9 times as tall as me. Since I should have been about five feet ten to six feet tall, that boy should have been about five feet three inches to five feet five inches tall when he probably should have been eighteen years old.
A few months after the yearbook photos were taken, I saw a boy, whose age I wasn't told, who was inches taller than me. Standing beside him gave me the creeps because of how cute and young he looked. It seemed unnatural, like he was a young child who had been enlarged by a mad scientist. This boy was inches over six feet tall, and possibly younger than sixteen.
In 1994 I went to a diner in a small village in southeastern Pennsylvania to pick up lunch. And, probably in the summer, there was a boy working there who was about five feet tall and looked about twelve years old. He was blonde, and as cute as most twelve-year-old child acters - I guesss he was approximately as cute as Chad Allen who was born June 5, 1974 https://www.imdb.com/name/nm0020354/?ref_=nv_sr_srsg_0 at the age of twelve, like in the first season (1986-87) of Our House.
ONe evening I was having dinner at a restaurant in that village, and that boy was one of the waiters who brought our dinner. One of my companions asked if he was old enough to work there. He said yes, he was sixteen. And my companion quite understandably said: "I don't believe you! You're so cute! you look like your twelve! You're so cute!".
Of course there are exceptions to age limits for children working in the family business, but would two different places in the village be operated by the boy's family? And I think he would have had to be age sixteen to work in a restaurant that sold liquor. Certainly nobody would take his word that he was sixteen but would demand documentary evidence to show to restaurant inspectors.
The Swiss actor David Bennent, born September 9, 1966 https://en.wikipedia.org/wiki/David_Bennent, is famous for the role of Oskar in The Tin Drum (1979), but I remember him as the elf leader Gump in Legend (1985). Legend (1985) should have taken a couple of months to film, but has a filming date of 24 March 1984 in IMDB, when David Bennent would have been 17 years, 6 months, and 15 days old https://www.imdb.com/title/tt0089469/locations?ref_=ttrel_sa_4. But Gump looks about five feet tall and about twelve years old.
Part Three: Some Historical Examples.
Here is a link to a story about Adolf Hitler's last public appearance https://historyofyesterday.com/alfred-zech-18a89a8d4ae6, decorating soldiers on 20 March 1945. Alfred Zech, the small boy wearing a black uniform in the photos, was born 12 October 1932 https://en.wikipedia.org/wiki/Alfred_Zech, and so was 12 years, 5 months, and 8 days old. And the boy in a grey uniform beyond Zech isn't noticeably taller. That boy was Willi Huebner (1929-2010) https://rarehistoricalphotos.com/wilhelm-willi-hubner-1945/ who should have been 15 or 16. In the photograph with Artur Axemann, the top of Hubner's helmet is level with Axemann's shoulder.
Sabu Dastigir (27 January 1924-2 December 1963) was a child star. https://en.wikipedia.org/wiki/Sabu_(actor) The Drum or Drums (1938) was first released on 7 April 1938 https://www.imdb.com/title/tt0030082/releaseinfo?ref_=tt_ov_rdat when Sabu was 14 years, 2 months, and 11 days old, so Sabu should have been thirteen when it was filmed. Drummer boy Bill Holder (Desmond Tester) always looked approximately the same size and age as Sabu's Prince Azim to me, but Desmond Tester was born February 17, 1919 https://www.imdb.com/name/nm0856469/?ref_=tt_cl_t_6 and so was eighteen when the movie was filmed.
I have measured my head with a ruler and a mirror as being 9 inches tall. Assuming that most people should have heads that tall by the time they go to school, the heights of people in old phtographs can be measured using their heads, among other items in the photos.
King Edward VIII of the UK (22 June 1894-28 May 1972) came from a wealthy family who could afford to keep their children well fed.
In this picture https://en.wikipedia.org/wiki/Edward_VIII#/media/File:Prince_of_Wales_9.2_inch_gun_HMS_Hindustan_Flickr_4454627308_cd3c9739e8_o.jpg of Prince Edward as a midshipman on HMS Hindustan in the summer of 1910, about the time of his 16th birthday, he appears to be about 6.4 times as tall as his head, though his cap makes it a little uncertain. That should make him about 57.6 inches, or 4 feet 9.6 inches, tall.
He was invested as Prince of Wales on 13 July 1911, aged 17 years and 21 days. In this photo of him in his regalia as Prince of Wales he appears to be about 6.4 times as tall as his head, or 57.6 inches, or four feet 9.6 inches, tall https://www.alamy.com/stock-photo-the-prince-of-wales-later-king-edward-viii-in-his-investiture-robes-37418803.html.
In this photo from 1920 https://www.posterazzi.com/the-prince-of-wales-later-king-edward-viii-in-1920-edward-viii-edward-albert-christian-george-andrew-patrick-david-later-the-duke-of-windsor-1894-posterprint-item-vardpi1957746/, when aged 26, he appears to be roughly about 7.5 times as tall as his head, or about 67.5 inches, or five feet 7.8 inches, tall.
According to this site https://www.celebheightwiki.com/duke-of-windsor Edward VIII was 5 feet 6 inches tall at age 78, so maybe taller as a young man. That indicates my photo measurements were fairly accurate, and that he grew a lot after his sixteenth & seventeenth birthdays.
Here is a link https://www.loc.gov/resource/ppmsca.34478/ to a photo of General Grant's cavalry ecort at City Point, Virginia, March 1865. The five bugler boys on the left look about ten to twelve years old to me. But this photo is reproduced in the multi volume Photographic History of the Civil War (1911), and i think I remember the caption describing the bugler boys as being fifteen and sixteen. That gives the impression that it was rather common in 1865 or 1911 for boys those ages to be only shoulder high to men.
Here is a link to a photo of General Custer's staff at Winchester, Virginia, December 24, 1864. https://www.flagcollection.com/tour_listview.php?CollectionTour_Code=groupings_custer
The soldier on the far left behind the flag lance is said to be Bugler Joseph Fought. According to his military records Fought enlisted in 1860 aged fourteen and four feet eight inchs tall, again in 1865 age 19 and five feet three inches tall, and again in 1868 age twenty two and five feet eight inches tall, thus indicating that he grew a lot after his nineteenth birthday and was quite small when he was sixteen.
And my reading about teenage civil war soldiers indicates that many of them were quite small for their ages compared to boys those ages that I knew.
So if Alendyias allows for the fact that most contemporary persons think that five feet is very small for a sixteen year old boy, and convinces them that it is a normal height in the time and place the story is set in, thereis no problem with making Ryker siteen years old and about five feet tall.