By default, all themes UI use the resolution 1600*900 (The resolution of my laptop)
Do not use the scale feature of blizzard, because this feature has a direct impact to your FPS.
You can change the scale of each UI element (window) to fit your resolution.
From the desktop tool, you can change the scale of a selected window or use the Global scale to change the scales of all windows.
I made this video for you to better understand the concept.
http://www.youtube.com/watch?v=lwsbbT0f3Ws
Cheers