1. the buddypress styelsheet is in extend\buddypress.css. The option you selected loads this.
2. bbPress is meant to be theme independent, so there should be some styling being loaded by the plugin.
Do you have a url we can see to check for you?
The theme demo is running BuddyPress but the forums in BP are from bbPress. See http://buddy.livingos.com/pure/forums/
In the buddypress style sheet we are forcing the avatars to be 20px, because as you have experienced the plugin default behaviour is for them to be larger.
The line of css is:
table.forum td img.avatar {
margin-right: 5px;
width: 20px;
height: 20px;
vertical-align: middle;
}
If you have a url we can have a look.