I have a shapeshifter character whose power-set is relatively grounded. They can only change into other human forms. They can basically impersonate anyone they want, so they use their powers primarily for spying purposes. But I’m struggling to imagine how exactly they’d be able to change their height/size. And I’m also unsure as to what limitations there should be. Should they be able to grow beyond 50 feet tall or should there be a limit? Should they be able to impersonate people of different genders? If so, how would they shift around their sexual organs? Should they be able to impersonate people of different ages? Even a newborn? I know that shapeshifting is fictional, but surely there are ways to make it plausible in theory. I am aware that variations of this question have already been asked on here, but I didn’t fully understand a lot of the answers given, as they were very technical.
You can't change their mass, unless they can quickly absorb or eliminate water.
I'd treat the shapeshifter much like an Octopus; which has no skeleton. The only hard part is the beak; much like a bird's beak. But no skull, the body has an outer layer of muscle.
So it can change color, contort itself into crazy shapes, etc. Maybe it could even simulate hair, if the control is fine enough, or maybe it can extrude hair and re-absorb it.
It could appear to be fat, but it would just be inflating itself.
The same thing for genitals, and breasts. It would be able to simulate either male or female (or it could be "smooth around the bend").
I would guess it cannot be a baby, maybe the size of a 12 year old, and no reason it couldn't be 7 feet tall, fat or thin or (in appearance) muscular.
Octopi have incredible ability for mimicry in their appearance, they can literally make themselves virtually invisible by making their body look like the background; even a rough and non-uniform background. They can make themselves look like sand, like rocks, like plants, like a rough surface, like smooth one. Colors and texture can both be copied.
I don't think it is a stretch for your shapeshifter to look exactly like somebody they want to look like, and if they have the fingerprints, to copy them also.
They may also be able to look like a trashcan, or a section of a brick wall. Perhaps some medium sized item in a store or office, like a desk.
I think your size limitations are a good idea. They cannot impersonate an aircraft, for example. And I would not give them any powers like flight, or impersonating a working machine like a car or a blender or even a bicycle. Just the appearances of them. But they might impersonate a living lion or panther, or a gorilla. Not an elephant or blue whale, not a sparrow, but animals reasonably within the weight range of say 80 pounds to 500 pounds (remembering the shapeshifter would just be hollow when impersonating something large).
I'd go with the ability to extrude and reabsorb "hair". It may be a stretch, but I don't think it is a deal breaker at all.
I said this in a comment but have decided to actually answer. There is the law known as the Conservation of Matter. If you want a realistic shapeshifter then they need to rule this law. When shapeshifting they cannot add or remove atoms from their body. So, they cannot become the size of a baby unless they want to die(That is my idea, they will probably just die from losing a lot of body mass). If all you want your character to do is change their appearance then you don't need to do anything. For a shapeshifter to change into another form, like a bull, then they would be a smaller bull. If it is a dog, then a bigger dog(though dogs can get massive).
The other answers by @Amadeus and @Martamo have detailed how through conservation of matter, the shapeshifter wouldn't be able to grow super large or super small, nor would they be able to change their age, only changing how they appear. Therefore, I won't go into detail about similar topics. However, part of your question is how a person could shapeshift and to what extent they could, for instance whether they could change their sex organs or not.
If shapeshifters were possible in real life, they would probably implement the use of stem cells. If a person or other organism could change all their cells into stem cells, rearrange them, and then convert the stem cells back to regular cells, then the person would be able to change face details to look like someone else, including color of eyes and hair, as well as changing their reproductive organs. The person would still be working within the limitations of their number of cells, and of course it is impossible in reality for humans to convert all their cells to stem cells at will, but stem cells would be an important part of shapeshifting in a hypothetical situation.
So while a shapeshifter wouldn't be able to grow 50 feet tall or change into a newborn, they could implement stem cells to change organs and features.