I like a 2 MOA dot for all around use. One thing about a red-dot sighted pistol is that your effective range is extended; unless you only intend to shoot fast and up close, there's no need to forego the greater precision a 2 MOA dot allows.
On a pistol? I shot with a 3.25 and wouldn't have wanted it any smaller. But I never tried a smaller one either so can't say for sure.
