Donna, that's one of the reasons going to shows is helpful as you can see many plants at once and make assumptions that usually hold true.. if a tiny or small plant is in bloom it often will stay manageable.. if you see huge 18" bare root sections, you can assume it will be large. Many of the mail order will have a soda can or something else next to the plant to give you an idea of its size. If you are looking for mini's I'm happy to give you the names of my few that have stayed window sill size. Also, I know marble branch farms has sub compact and compact plants on their website.
