[Freesteyelz]

I've always edited the postbit and/or postbit_legacy templates and defined the width/height of the avatar. If you don't mind doing that then...

Find:

<img src="$post[avatarurl]" $post[avwidth] $post[avheight] alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" />

Replace with:

<img src="$post[avatarurl]" width="x" height="y" alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" />

Replace "x" with width size and "y" for height size. It doesn't matter what size the image upload is as it'll be forced to display your specified dimensions.