Nos operamur, te ludere
"Life is more fun if you play games." - Roald Dahl
For ease, I'm using the standard 2.5" x 2.2" used by Game Crafter. It'd be a headache to create a prototype then recreate it for mass printing.
I don't think there's a standard -- you will most likely have to re-make your files for printing, even if they're close.