Most effective postcard sizes

I'm wanting to put together a direct mail campaign using postcards. I know a lot of people talk about the effectiveness of different postcards sizes so... what is the most effective size?

I'm a fan of 6' x 6' myself. They stand out among the stack.

I don't know which size is the most effective (I'd be interested in knowing that myself.) Just thought I'd offer a pertinent fact (which you may already know): in order to qualify for the special postcard rate in the U.S.(.23), it cannot exceed 6" X 4 1/4"

I use 6*6 , thats best and fixes in hand well and is lot ceaper too