Hello,

You should be able to control the avatar image dimensions in Site Manager -> Settings -> (select site from drop-down menu) -> Community -> Avatars.
There are several properties, like:

Avatar max side size - maximal size of user avatars; if one or both sides of the image are longer, the image will be re sized so that the longer side's size matches the entered value; if 0 is entered, 'Avatar height' and 'Avatar width' values will be used instead
Avatar height - if 'Avatar max side size' is set to 0, images will be re sized to this height
Avatar width - if 'Avatar max side size' is set to 0, images will be re sized to this width


You can also modify this control directly in the ASPX code found here- ~/CMSModules/Membership/FormControls/Users/UserPictureEdit.ascx

Best Regards,
Edward Hillard