I actually have a 1440x900 screen.
Overlay: 720 x 576 (I think... I set it something around here, the important part is to get an aspect ratio close to 16:9.)
Background 1440x900
This will have the window take up about less then half of the screen while retaining the old macro look. You can also maximize it now without it looking like the rear end of a Galka who had a run in with a Rusty Tail Cutter.
Go to the config once you've started up and change the aspect ratio to match that over your new setup. It might not be perfect, but this setup worked for me.
The key thing behind all this is mainly to have your background resolution be larger then your overlay... it simply makes things look better.