BuddyPress.org

I’d like to make my activity-avatars 50px rather than 20px. The site I’m working on: http://www.asifaportland.org/activity/
The offending lines in the source code seem to be:

‘.widget #message-threads img.avatar {

height:20px;
width:20px;’

… But I can’t figure out where to change this. There’s a similar discussion here ( http://buddypress.org/community/groups/creating-extending/forum/topic/how-to-increase-activity-avatar-image-size/ ) but without resolution. I’m stumped. Thanks for any help.