You will find the corresponding option inside:
Dashboard >> Reign Setting >> Community Settings >> Avatar Setting
You will find the following options inside it:
Avatar Thumb Size: Changes user and group avatar to selected dimensions in an activity, members and group lists.
Avatar Full Size: Changes BuddyPress user and group avatar to selected dimension in user and BuddyPress group header.
Avatar Max Size: Changed maximum image size a user can upload for avatars.
Default User Avatar: Upload an image that displays before a user has added a custom image.
Default Group Avatar: Upload an image that displays before a custom image is added to a group.