If one handmade greeting card doesn't seem like 'enough' to you, how about a set of three, with handmade envelopes (not hard to do) all in a baggie? Those would be great. As far as size goes, check with the PO for what mailing sizes are. Postcards (my hobby) are 3 1/2 by 5 1/2 inches. 1/4 of a sheet of standard paper (cut in half then fold) would be 4 1/4 by 5 1/2, and you could cut that down slightly to fit in an envelope.
My mom, bless her departed soul, used to make handmade cards with wildflowers fastened on the fronts somehow, and small painted embelishments. I still have some that I treasure.
I often leave antique postcards, sets of 3 or 4 along with postage. It's not a sig item, but it's not a McToy either.