Here are the maximum sizes for Lettermail in Canada and the US:

Within Canada: 245 x 156 x 5 mm
To the US and internationally: 245 x 150 x 5 mm

USA

Within the US and international: No more than 6-1/8 inches high X 11-1/2 inches long X 1/4 inch thick (= 155mm x 292mm x 6mm)

I'd vote for a print size that is no larger than the maximum we can send lettermail in each of our respective countries.