I like mine oversized, but I don't have a specific size and I often use scraps for the whole shebang so they're all a little "unique". They kind of end up whatever size they end up - most are probably around 8" square.
If they're too large and getting into food (which does happen) I just fold them in half or fold down a corner and use them that way. My mom always had tiny potholders and I hated them, so I make BIG ones!
I make square ones or sometimes slightly rectangular ones. Mostly because that's easiest, and I consider potholders to be utility items, so I don't really fuss with making them special. Sometimes I will even just add a new "cover" to one that's gotten stained to the point it looks unsanitary, or a burn that has exposed the batting. I'm either lazy or thrifty, you decide! LOL