Christer, the changing of user widget avatar's size is considered as a custom code modification.  I can not provide you with detailed instructions. All I can just show you the direction. You will need to make changes  in the following files: /ow_system_plugins/base/bol/avatar_service.php, /ow_system_plugins/base/components/avatar_user_list.php, /ow_system_plugins/base/components/user_list.php.
 
If you are not familiar with the code modification or plugin development, it would be better for you to find a developer who can help you. You can do it here: http://www.oxwall.org/market/specialists.
Also you can apply to other members for the assistance in the custom code modification forum, probably some community member already have a solution.

Ty Taissa I will repost it there.

hmmm as far as I can see you do not need to change the php to change the size of the pictures; all you need to do is to use a chrome/FF F12 debug/dev mode, identify those styles that define the size of the DIVs which are parents for the user pictures and redefine them in the custom CSS.

of course it is tedious work but it can be done without modifying the php code.

Taissa is right, if you don't want the avatars to be blurry when you increase them to 200px you need to do the changes in php files first and then in css.