At the In-n-Out by work, they'll only do up to 4x4s unless there's very few customers (which never happens... actually, I imagine they'll do more if you go inside instead of through the drive-thru), and the biggest I've ever heard of being purchased was a 9x9.
There is a giant Creme Egg? Eating the normal sized ones with a friend the other day we imagined how awesome a giant one would be