How to find the exact size a button is going to get?

[Toralf Lund <toralf procaptura com>]

For various technical reasons I won't go into right now, one of my Gtk 
apps need to know what size certain buttons it is going to create will 
get, before it actually creates them. Is there any way at all I can 
calculate this? The buttons will contain pixmaps, and I have the image 
data for those, so I actually know the size of the "contents" of the 
buttons, but the total size of a button is somewhat larger as 
(apparently) borders are always added,  so how do I find the size of 
these?  The border associated with the "border-width" property is not 
the one I want, I think, but rather extra borders that magically appear 
based on the the style or theme or I don't know what.

Help, anyone?

- Toralf