In America basic foundation training can happen at local studios anywhere or at the company-affiliated or other elite schools. Just about anyone can train in ballet, even at the elite schools in the general/"no audition" tracks, and where focused professional training starts in the tweens or teens when students are allowed to live away from home. It's not like at the Vaganova Academy or Paris Opera Ballet School where their sole purpose traditionally is to train a very small number of students who meet strict standards for body type, proportions, flexibility and other physical attributes and for whom they predict the "correct" body past adolescence to become professional dancers. (Now they have a sub-business in attracting international students who will write college tuition-sized checks to subsidize their schools.)
If he were an American, chances are that if he survived the Creative Movement recital in which he was the only boy, and they stuck a bunny tail on him, or bypassed that and started later, Vasiliev, if he became serious in ballet and showed promise -- or tagged along for auditions with friends and got in -- would have auditioned for summer intensives, and as a boy, likely would have gotten at least partial scholarships, and then woould have been invited for training in a year-round professional program, leading to a contract. If he attended SAB, chances are his musculature would have been different, simply from the training he would have gotten. There have been plenty of men at NYCB and any number of SAB students dancing around the world who are shorter virtuoso dancers: body type is much less of an issue in America than in Russia, and in Russia, he received elite training and won Moscow and Varna and joined the Bolshoi, so it wasn't an obstacle there either, where the standards are a lot more limiting.