Them's the ones, Granny!
The 3x4 and 4x6 are closest to what WalMart sells. I use both sizes but if I were to only go with one I'd pick the 4x6. I don't think you need anything bigger than that. For color I'd stick with white or ivory. It's easier to see through the lighter-colored fabric what stage the blossoms are at or if any fruit has set yet.