I have trouble setting a custom size for my 50x50 icons. I make a mix of custom icons - some added through default services (for example Facebook) and some through custom services. I use my own icons.
The problem is: I can't set the custom icon size. So default service's icons are shown with the size of the custom icons and the custom service's icons are downgraded to 32x32.
A suggestion: add a textbox, where you can specify the dimensions. If the field is empty, use the values form pull-down menu.
Google Page Speed also complains about IMG tags not having sizes specified. Default services could also have sizes specified as custom services do.