These run in junior's sizes.  I figured this out by purchasing my first from a brick & mortar store and realizing a 7-8 was considered a "medium"  They work great as a rash guard shirt and should be ordered to fit like a glove (pardon the pun)  Great swim/surf shirts if you get the right size.

Here's a cheat sheet to their sizing:
Size XS 3-4
Size S  5-6
Size M  7-8
Size L  9-10
Size XL 11-12