It will allow you to insert a picture of almost any size, really, but you can usually tell at a glance at other members' avatars if yours matches. :) If not, there are several choices:
1. Right-click on the picture and go to inspect. It will show the dimensions to the right.
2. If the picture...
Oh, my! Avelina Sedai, that is beautiful! I really liked your old avatar, too, but this one is so lovely... I love the style in which it was done. Welp. *Cracks fingers* Time to enter the adaptation phase. :lol