There was another thread on card sizes recently. Check it out here:
Forums at Splitcoaststampers
My current size that I have been using alot is 3 1/2 x 6 1/4 (fits in a small business envelope.) I've made some fun stuff with that size and the envies are cheap so that's a plus!
My second favorite is 4 1/4 x 6 1/4. The only problem with these is that all of have is 100 banana envies for them so I'm limited to color combos. I'm fine with banana but Momma doesn't like yellows so I don't tend to make as many of these as I would like to.
I also like square card size - whatever the size of envy is that SU sells...5 1/4 x 5 1/4?
Of course you can never go wrong with the regular 4 1/4 x 5 1/2. Most cards that I make are made in that size.