When making print and play games I try to make one file everybody can use instead of one for US and one for Europe. To have one working document we need to make the prints within the shared space of the formats. The smallest shared space between US Letter and International A4 is 210 mm width x 279 mm height. (8.27″ x 11″).
And use a margin that is safe for most printers which could be 0,25″ or 6,5 mm.
Remember that your art needs to be saved as CMYK and not RBG.