Around here, North Carolina, you see a lot of trailers set up for both side and rear service. I've had several trailers over the years and I tend to prefer the smallest(shortest) trailer possible to handle the task at hand. Smaller is easier to pull and park, and cheaper to pull as well which may be important with gas at $3 a gallon. You don't really need much of a trailer if your menu is limited to smoked meats and a few sides and drinks. Mobile kitchens that will be used for various types of cooking for potentially large crowds work best with a little size. Never understimate the need for refer capacity and flat work space. I see table space shorted frequently for additional cooking equipment, but all the cooking equipment in the world won't help you if you don't have any place to stage the cooked product.
I do a fair number of upscale type parties and such where I need the table space to prepare trays and such. For a few months a year, I can do it outside under a tent, but most of the time it's too hot, buggy, or cold, so the inside space is really nice.
I currently own a 17' trailer set up for pig pickins and cook out type events and feel it has plenty of space and it could easily handle a fair or race type events as well. Also, I have access to a 28' mobile kitchen I built and sold to a relative, and while the space is nice, I'm glad I don't have to pull it every where I go. Also, with it I try to be first on the scene to make for easier parking, and I'm usually last out too.