Yeah I agree with what Doug said about waterproof, especially since I live in the Northwest!
Are these postcards waterproof? From the picture I could tell they were at the very least glossy and probably more durable than regular printed paper, but Doug brings up a good point about smearing ink.
I thought about the size issue too.
There's advantages to both sizes:
Business card size you can always have a couple in your wallet and be ready under any circumstance.
4x6 is more noticible on our low wipers and provides room for a bigger, more interesting image on the front, more detail on the back.