crate size for mini

I read the article about crates and training on this site. I'm still confused about what size crate to get. We will be getting a English F1 mini doodle in a few wks. It seems these dogs get to be about 25-35 lbs and are considered "medium" in size. The 30" dog crates indicate they are for medium dogs, 36" and up for large dogs. Can anyone advise which size is better or advantages of one size v other?