[Cryo]

You can't set a minimum without coding or through a plugin. Unfortunately, I am not sure how to do this, it would probably take place during some of the post-upload processing. However, you can edit the template so that it stretches whatever they upload to 100x100. In your postbit_legacy / postbit templates, search for...

postbit_legacy...

<img src="{vb:raw post.avatarurl}" alt="{vb:rawphrase xs_avatar, {vb:raw post.username}}" title="{vb:rawphrase xs_avatar, {vb:raw post.username}}" />

Replacing it with the following...

<img src="{vb:raw post.avatarurl}" width="100" height="100" alt="{vb:rawphrase xs_avatar, {vb:raw post.username}}" title="{vb:rawphrase xs_avatar, {vb:raw post.username}}" />

postbit...

<img src="{vb:raw post.avatarurl}" alt="{vb:rawphrase xs_avatar, {vb:raw post.username}}" />

Replacing it with the following...

<img src="{vb:raw post.avatarurl}" width="100" height="100" alt="{vb:rawphrase xs_avatar, {vb:raw post.username}}" />

It won't require you to have a 100x100 image uploaded, but it will stretch everything to that.