You've already discovered that your feet swell when you run, thus the need to go for a size that is a bit bigger than your normal everyday shoes. But how much bigger (the length between your longest toe and the inside front of the shoe) is very specific to you. And some people have wider feet that others. As lenzlaw mentioned, the guideline is a thumb press from your longest toe... You don't want a shoe that fits too loose - that can lead to blisters and just generally being uncomfortable after a few miles. The shoe has to be loose enough that you can easily wiggle your toes, but tight enough that your foot doesn't move around in it. And with that explaination you can see why shoe fits are a very personal thing...!
-bill
http://runningthrutime.blogspot.com
(what do I know...? I'm just getting old...)