It's clear that it'll always depend on resolution ... I mean you don't need larger smudge brush on a picture with small resolution. I once tried out 2500x3000 somehow and to work accurate with this resolution I'll need a brush up to 150 diameter, but this goes with a speed that is not acceptable any more. It's no earthquake to work with smaller resolutions, in no way, but it'll make things much prettier to be able to create high resolution images as resolution is vital to the quality of printing. I'm actually taking part in a competition where the image should be printed out in A1-A0-size and actually, that goes not so well with 2000x2500 pixels!