is there a way to set the hight but have the width be a %
is there a way to let the user point to a URL for the avatar?
1)Not with the current setup, I have another admin setup page that allows you to specify your own values, I suppose you could use % in that, download it here ...
2)Not with the current setup, you could adapt the admin upload feature to allow users to upload I suppose.
There is a version of the avatar mod by cbovis that allows users to specify their avatar path, that may be of more use to you.