WordPress.org

Can I choose between usernames/full names?

Yes, you may choose to display your users by their username, or full names, or to hide both if you want.

May I change the tabs colours in order to have them like my blog ones?

Of course, you may customize the colours of every part of the user boxes

May I change the dimensions of the user pics?

Yes, you may choose your personal pic width and height and every pic will be resized according to them.