Personally, I like the
Galco SB9 Dress Belt in 1 1/4". It doesn't look like a "gun" belt, and is a sturdy two layer leather (actually two layers of leather stitched together) so it is very stiff and gives good support.
I like the tan, but tend to wear black as it hides holster clips.
The main thing about dress pants is that the loops are only so big. 1 1/4" is no problem. 1 1/2" probably is OK too. Anything larger than that and it might not fit the loops of some pants, esp. dress pants.
In general, I think it is good to pick a size and stick with it. When you order holsters you want to pick a clip/loop size for your belt. If you pick everything the same and stick with it you'll have more options. While you can often wear a holster designed for a wider belt, it can feel 'sloppy'.
-john