What is wrong with the 20" folding bikes? A lot more options out there. FWIW my wife rides a 24" wheeled Huffy Cruiser, a 20" wheeled Raleigh Twenty and a 16" wheeled Dahon, as well as 26" wheeled Raleigh Colts. Personally if you are going with the folder I would go the 20" wheel route unless there is some compelling reason to go smaller or larger.
Aaron