Take the resolution you're running at, divide each dimension by 100 (to get inches), then multiply by 25.4 (to get millimeters) (the net result is approximately what you'd get by dividing your resolution by 4).
Then put a DisplaySize line in your Monitor section with those two numbers.
I run 1280x1024, so approximating the numbers, I'd use 320 and 256 for my DisplaySize values. The line looks like DisplaySize 320 256 in my Monitor section (or would, if my driver was reporting the wrong DPI -- it's not currently).
You could also try 4363 -- maybe some of the other bugfixes helped this issue too?