You can do them on anything you want. Any size canvas or use the pattern for cross stitch or anything! I didn't see any specifics on canvas count on the leaflet.
No offense, I'm not trying to be rude, but did you check inside under the individual pattern instructions? While it is true you can choose any size canvas, the finished product varies greatly. I have NEVER seen a published plastic canvas pattern that didn't indicated what count canvas is preferable to use, along with finished dimensions.