Let's talk about the fit of leotards! You want leotards to be comfortable, but also skin tight so the gymnast has the ability to move freely and the coach can see the lines of the gymnast.
Most sizing of leotards is fairly simple, they use sizes like:
Child Small (CS), Child Medium, Child Large, Child X-Large, Adult X-Small, Adult Small, etc.
If you are ordering online, often times there will be a size chart. To find the best size for you simply measure the gymnast and see which measurements are the closest to the measurements on the website to find the best size! Here is GK Elite Sportswears size chart: http://www.gkelite.com/sizecharts
Other popular leotards are Alpha Factor, Snow Leotards, K-Bee Leotards and Motionwear.