This folded greeting card might be as close as you're going to get. It's 4" x 5.6" overall, while a postcard is 4.25" x 5.6" I don't know what you had in mind but does that work for you?
Thanks, this is what I found too. I wasn't sure of each side was the size listed or overall. It is for a returning customer so I will check it over with them.
