Well, IMO, just like a barbell, one size body bar wouldn't be appropriate for working all muscle groups. I guess it depends on what kind of training you plan to do with it, and your level of strength.
I have 9, 12, 15 and 18 and use them in lieu of barbell or a (balance) stick, not dumbbells. Dumbells are great because they force each side to work independently. Alternately, barbell/bodybar is great because it can help you with balance and stabilizing muscles.
So if you are planning on using balance/endurance vids with it like the body bar series, I'd go lighter: 9, 12, and 15 if you're really strong. If you want to use them for strength-type work like firm and cathe, go on the heavier side. But if you've never used a barbell/bar before, there is a learning curve so maybe you should get a lighter and a light-moderate one (9 and 12?). Maybe there is a sporting goods store near you where you can test them out.
Hope this helps, good luck,
Kathy