I have found that the determining factor in leotard sizes for both children and adults is the "girth" measurement, which takes into account the body length, which can differ significantly among children or adults of ostensibly similar height/body size. If the girth measurement is correct, the elasticity of the leotard in the other directions is sufficient for a good fit. Most brands list the girth of the leotard with the other sizing information on their websites, and this bit of information makes online shopping less fraught with uncertainty
Girth is usually measured by holding a measuring tape on top of one shoulder, passing the tape down the body and between the legs, then up the body to the same shoulder. There are diagrams available online if you search "girth measurement leotards".
Good luck and happy shopping!