BuddyPress.org

https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/how-to-put-avatar-upload-in-registration/#post-86699

To change the size of header avatar, you will need to go and change it in your CSS.

`div#item-header ul img.avatar, div#item-header ul.avatars img.avatar {
width: 30px;
height: 30px;`

adjust to whatever.