2 X 2 Holders

I wouldn't have a hard-and-fast rule. If it's a very thick coin like a British pound or a cartwheel penny, you're going to need more than a couple of mm or you'll risk stretching the film too far and breaking it. And if a coin has an edge inscription, you might want to consider leaving enough room to enable someone to read it.

Personally, I keep a stockpile of assorted 2x2 sizes and use whichever is the closest best-fit, even if the size is out by more than just a few mm. Using a giant 2x2 to hold a tiny coin looks unprofessional, and the coin can pick up "cabinet friction" if it's rolled around in there too much, but otherwise it does no harm; I've done it numerous times when I've had coins to seal up and couldn't be bothered going down to the coin dealer to buy some more... or if the packet of 100 2x2s I did just buy turned out to be the wrong size.