The real answer to the question is "whatever the maximum required to display all the kcontrol widgets". That being said, the 640x480 default probably came from KDE1 when VGA was the new standard. The fly in the ointment is the new 10" mini laptops -- I don't know what their resolution is, but I think we could safely do 800x760 as the default kcontrol size.
What we need to do is just run kcontrol and find out what the max size needs to be using kruler. I'll take a look tonight and see what I come up with. I'm betting that 800x760 would do while still preserving usability with the 1320x768 res monitors.
Sounds nice. You also have to knnow where in the sources to make the change. Somewhere in tdebase/kcontrol? :-)
Darrell