You can use this CSS code. Put it in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea, save it delete all caches and do Ctrl+F5 on forum front:

#wpforo #wpforo-wrap .wpfl-1 .wpforo-post .wpf-left .avatar {
 width: 100px; max-height: 100px;
}
#wpforo #wpforo-wrap .wpfl-1 .wpforo-post .wpf-left .author-avatar {
 max-height: 100px; max-width: 100px;
}

Just don't make it more than the left section. I can't support full restriction of post layout. You should find some CSS specialist for deep customization.