"Optimal" full screen window size

I am sure this must be a FAQ, but I can’t see any reference to it
anywhere. Is there a way with SDL to find out what the current screen
width and height is so that I can use this for my full screen mode, to
avoid changing resolutions?

I could do some Xlib calls to find out the size, but I’d like some
portable way if it exists.

Thanks,

Cameron–
Cameron McCormack
// @Cameron_McCormack
// http://www.csse.monash.edu.au/~clm/
// icq 26955922