if your screen is already at max res, 1024x768 (a very low res), then there isn't much to do.

if you right-click the desktop / properties (personalize)/screen settings (change resolution) ...
can you drag the resolution bar further to the right? what is the highest it will go?

(if you don't know...)
by increasing the res, you increase the number of pixels on your screen... this will allow wow to have more room to render your game screens.. but it will also 'shrink' your desktop icons. they don't actually shrink as they are still 32x32 . it's just that with higher res, 32x32 renders into a smaller region

higher res == cleared picture
BUT
higher res == more cpu/gpu intensive