To replicate...
When running SL client full-screen choosing "Set Window Size" from the menus and picking a size (e.g. 800x600)
Current behaviour...
the client switches from full-screen to windowed but does not resize to the new dimension, in this case 800x600, but something a little smaller.
Workaround...
Choosing another size and then choosing the required dimension (800x600) again sets it to the right size.
Expected behaviour...
I believe this should set the correct width and height.