I just mailed my postcards out. I took 11x14 paper and cut it into 5.5 x 7 pieces and made my prints. The post office didn't mention if they were too large or not. They cost me the regular letter amount of $1.00 to the USA and $1.70 International. Canada hasn't had a cheaper Post Card rate for decades.
Rob - I received your card yesterday so must be within regulations. Very nice paper btw and a great image for the paper.