In the admincp,... go to the usergroup manager and click on the user group. About half way down you will see various options for avatars including dimesions and file sizes.
damn thanks... now how come my registered users are only allowed to have size 128x120, but one of my members runs a 200x200 avatar?
Because the restrictions came into place after the user uploaded their avatar. You'll have to change it manually via the ACP or kindly ask them to change it themselves