I had the fortune of getting a full set of Competition X kites used right as I was getting into buggying. (The XS to the XXXL)
Being in Ohio we tend to get very spotty finicky winds, and most days are in the 4-9 MPH. I'm buggying on grass and a big guy, so I tend to need a
slightly bigger kite than most.
I use the XXL (7.1m) quite a lot, more-so than any other in my quiver I'd say. Though the XXXL (9.7m) is a bit more useful in the 4-5 mph range.
With almost no wind you can get either the XXL or XXXL in the air, and they pull like crazy (and steer like a truck) when you turn them. But you
really need 3+ with the XXXL and 5+ with the XXL to get moving on a buggy.
A lot depends on how much you want to work the kite. I tend to use a kite that isn't quite enough to pull me until I start to do figure 8's and really
increase it's pull. This helps to keep me out of trouble (and being over powered!)
I've only gone airborne once, and that was about 2 feet up and 10 feet forward, not planned, using the XS in 25-30 mph winds. Funny thing was, I was
trying to get the thing on the ground when a wind gust hit! Limped for weeks.
If you can get a hold of a Comp used, I'd leap on it. They fly great, and have lots of power.
-Chip