What exact page you want to edit? Can you give the link to the portion of the edit required?
What I think you are asking for is
http://xyz.com/forum/memberlist.php
Then follow these steps:
Style manager > Edit templates ( of the style u are using) > CSS templates ( click for a list to load) > Memberlist.css >
Look for
Code:
#memberlist img.avatar {
max-width:
Put the value of max-width:180px (if that's the size you are looking for?) > Save.
Refresh your member list. Larger sized avatar would be available. :-)