The diameter of the test sticker I ordered is three inches. I think that's about the same size as the PCA stickers. The computer file should be high enough res to make a four-, five-, or six-inch sticker, but I don't know if the place I've ordered from offers those sizes.
This is the first time I've tried using this online outfit - StickyLife (no comments, please).
If anyone has had good experience with an online sticker place, let us know.