The title of this blog post is a trick question! There really is no "best" size. Hi! I'm Nikki, an educator for 20+ years. I teach all things rebounding on my YouTube channel. Let me share some rebounder size options and reasons why you might choose one size over the other.
Most rebounders have somewhere between a 32 and 48 inch diameter give or take. Based on my experience with thousands of participants, I would be more concerned about quality versus size. I'd rather a person buy a high quality small rebounder, than a low quality big one. Tension is the name of the game when it comes to rebounding, and in my opinion, is much more important than size. Read more about rebounder tension and body weight here. You want a rebounder with a smooth and responsive "bounce" or rebound, not something so stiff that it is jarring or something so soft that it is destabilizing.
Smaller rebounders are great if you don't have a lot of space or want something portable. Larger rebounders are helpful to those who are tall or have large feet. Most people adapt to whichever size they buy. You can view people of all ages and body types jumping on 39 inch rebounders on my YouTube channel.
My favorite rebounder is the JumpSport Fitness PRO Series. They make two sizes, 39 or 44 inch. Try not to worry too much about which size to buy. You're about to have a whole lotta fun, regardless of the size you purchase!