... I'd vote for a print size that is no larger than the maximum we can send lettermail in each of our respective countries.
Here in Germany we have different sizes/costs for what we can send as lettermail. The standard and compact letter sizes will just not hold a 5x7. To use this I would have to trim it to something smaller. Then the bigger sizes will easily hold a 12x16 inch print. So in this case lettermail is not a good restriction for a small print That is why I'd rather vote for biggest paper size is 8x10 and everything smaller is allowed and welcome.