Lucky for us I snagged a pamphlet and here are the dimensions:
*flat rate envelope priority mail- 12.5 inches x 9.5 inches all countries besides Mexico and Canada- $11.00 6-10 business days for delivery.
*flat rate box priority mail- 13.625 inches x 11.875 inches x 3.375 inches all countries besides Mexico and Canada - $37.00 6-10 business days for delivery (this is the one I sent).
*flat rate box priority mail - 11 inches x 8.5 inches and 5.5 inches all countries besides Mexico and Canada - $37.00 6-10 business days for delivery.
*NEW large flat rate box priority mail - 12 inches x 12 inches x 5.5 inchex all countries other than Mexico and Canada- $49.00 6-10 business days for delivery.
* NEW apo/fpo large flat rate box - priority mail- 12 inches x 12 inches x 5.5inches for apo and fpo address $10.95 //can also be shipped to non- apo/fpo addresses within the us for only $12.95.
clear as mud? hahahhaha :):):)