That sounds like a reasonable size for an arena for barrels/poles.
As far as the ground, that is personal preference. I personally like more dirt than sand. (maybe 2/3 dirt 1/3 sand, or 3/4 dirt 1/4 sand) The sand is helpful to keep things from getting so muddy, but too much sand makes it too "slippery" in my opinion.
Just make sure to have a DEEP enough layer, and the footing isn't too shallow.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.