I'm not sure if it's a straight thing or if it applies to some homos, but some people who are into feet like them small and .. well, I guess dainty. Since it's a means of humiliation for me I like 'em big; it's really hot when a guys shoving his foot in your face and his toes can still curl over your head as you're licking the middle of his sole. It's not necessary, but it definitely makes it feel like there's more power involved.
On hook up sites you'll see pretty often guys posting pictures of their cock next to beer bottles or soda cans, putting coins on the cockhead or - and I still don't understand this one - red solo cups. In the bios for the guys on foot sites, they usually list a shoe size for people who really care that much, but "Size 15" (the first guy) doesn't quite put things in perspective. Apparently some guys into feet have started using a similar approach, which I find pretty amusing ... and arousing.