Does size, length and bone structure matter for a gymnast?
It only matters in competitive gymnastics.
Doing gymnastics is an excellent way for anyone to develop good muscle tone, strength, agility, flexibility and balance. It is really the application of physics to the mass and form of the human body.
However, to compete "competitively" based on certain exercises and form, it may be observed from the "Winners" lists that the sport favors those with less mass and shorter lengths. This is expected, as many of the exercises involve rotations or other movements about the center of mass (COM). Heavier appendages are harder to control relative to the body COM, making it much more difficult to perform at an exacting level.