There is a FAQ on this subject here:
http://www.wowinterface.com/forums/s...ad.php?t=20917
That said, I could call 800x600 rez very low rez. But that's neither here nor there when it comes to the size of elements on the screen.
nUI is optimized for use on anything from a 17" wide screen on the low end of the scale to a 22" on the high end. At 17" some people will still find it a bit small and at 22" some will find it too big. You have to shoot for the majority on something like this.
Everything in nUI can be resized, but it requires editing the layout files to do it and, obviously, if you make everything bigger, then it's not going to fit inside the bounds of your monitor anymore, right?
Anyway, read the FAQ and see if it helps any. Other than that, there's really little I can offer. It's just not possible for it to work on every monitor size at every resolution out of the box. I can only do my best to make it work for as many as possible.