Genesis 6.4: "There were giants in the Earth in those days". And maybe that will be literally true in your story.
I can't help with the reproduction part, but I have thought of a way for 10 or 15 meter (33 to 49 foot) tall giants to be able to live on the planet Earth with a surface gravity of 1 g.
There was a circus male Asian elephant about a century ago who could walk on his hind legs for hundreds of feet, often with a woman standing on his tusks.
Wild African elephants have been observed and photographed standing on their hind legs and reaching up with their trunks for fruit and branches of trees.
So a large modern elephant could stand with the top of their head possibly about 6 meters or 20 feet above the ground, and able to reach their trunk about a couple more meters high.
Some prehistoric proboscideans were significantly larger than modern elephants. If they also could rear up on their hind legs they could have reached higher with their trunks, possibly higher than 10 meters or 33 feet but lower than 15 meters or 49 feet.
Giraffes are also quite tall even when standing on all four feet.
Fully grown giraffes stand 4.3–5.7 m (14.1–18.7 ft) tall, with males taller than females. The average weight is 1,192 kg (2,628 lb) for an adult male and 828 kg (1,825 lb) for an adult female.
A big giraffe standing on its hind legs should be able to reach well over 6 meters or 20 feet high. But I don't know if giraffes ever stand on their hind legs.
So a creature shaped like a bipedal elephant or a bipedal giraffe which was large enough might possibly be able to reach 10 meters or 33 feet tall. And that is about the limit in height on the planet Earth with a surface gravity of 1 g.
But there are Earth animals which are sometimes much taller than that, depending on their posture at the moment.
You can find many images on the internet of pods of sperm whales sleeping in an upright posture. Those pods consist of young whales and adult females, who rarely grow over 12 meters or 39 feet long. Males grow much larger.
I assume that large whales of several different species might be vertical while diving or returning to the surface and reach vertical heights of much more than forty or fifty feet when in that posture.
In Earth's surface gravity, animals can be reach or exceed 10 to 15 meters or 33 to 49 feet in vertical dimensions if they are partially or totally immersed in water or some other dense and common fluid.
I'll stick with water and let you try to immagine other flulds they might float in and what stories that might inspire.
Its not supposed to be an alien species, these creatures lived in huge caves under the earths surface.
Make those caves partially filled with water. The creatures could be descended from land dwelling bipeds or semi bipeds who evovled to live in the waters of those caverns.
They might normally float or tread water upright using their presumably webbed feet. And if they are related to humans closely enough, one of them might have been the father of Merovech.
And possibly they have evolved tall legs, bodies, and arms, so they can float in the water with their upper bodies in the air and reach up to the roofs of the caverns to pick objects - animal, vegetable, or mineral - from those roofs for some reason. Possibly biolumenescent organisms which they pluck so they can see when they dive deep for food or something lower in the waters of the caverns.
Humans who meet those giants in the caverns will have to carry their own lights, or rely on those hypothetical bioluminsecent organisms anyway.