I usually make cards the size to fit my envelopes - lately I bought couple sets of 5 1/2 x 5 1/2 cards with envies and I'm loving these..lot of space and you don't have to worry where to get an envelope ;)
Thanks for all the replies! I have been noticing that a lot of challenges uses the square size now. I think I am going to look for some square envelopes and then make the cards to fit! That is a bummer that the postage is more though!