[Kyojii]

<img src="{vb:raw prepared.avatarurl}" alt="{vb:rawphrase xs_avatar, {vb:raw prepared.username}}"  {vb:raw prepared.avatarsize} id="user_avatar" />

I'm not exactly sure where you change prepared.avatarsize, however if you change the above to the following you can edit the image size in that theme (doing it this way you would have to make the avatar size change for each theme individually). Maybe someone else knows where you can edit prepared.avatarsize.

<img src="{vb:raw prepared.avatarurl}" alt="{vb:rawphrase xs_avatar, {vb:raw prepared.username}}"  width="xx" height="yy" id="user_avatar" />

Be sure to replace xx and yy with the dimensions you want the image to be.