BuddyPress.org

For avatar, you can specify custom avatar size using bp_core_fetch_avatar() function,but in that case either you will have to edit the sitewide activity widget(Not a good idea) or use custom loops.

for images inside activity content,I will suggest,use css for that.You can target images inside activity using css and set a height/width.

It is just an easy workaround,I can recall for now.There may be other approaches using php.