They are more or less large pony sized. They are rideable, but I hear they are real flighty. There is a trainer around here that will train them, she's got pictures of her jumping with them, and other such. (I don't know her, and not sure how I came across her site) I just think it would be cool. ZB the Zebra prancing around with his (yes, it'll be a he) spikey striped mane, be my little punk rocker pony.
Apparently, you can buy them from anywhere between 1500 and 3000$ (females are more expensive), and aren't nearly as uncommon as I thought. There somewhere... I think in south Texas, that breeds and sells all kinds of exotics- including zebras, different deer types, camels, buffalo, etc.
(Not that I have that kinda cash lying around, but is not nearly as expensive as I thought).