People run all kinds of brakes for trials.....from normal v-brakes to mechanical or hydro discs.
Lots of people like mechanicals for trials, but if you're riding street where you might be side hopping, you run the risk of tearing up the rotors. Hydros are great and a little easier to control as long as they're well-bled and working properly, but a well set-up mechanical is just as good. The Magura 133 hydro rim brake is probably the #1 most used brake for trials. Most people run one on the rear, no matter what they use on the front....some people run it for both ends. They can be troublesome but they grab like nobody's business and there are lots of options for them. They sit out of the way and do a great job. If you want to run v-brakes, be sure to get some Moe's green pads for them and consider grinding your sidewalls.
Rotor size depends a little on what you're doing and where you're doing it. If you're riding natural, most guys use an 8" in the rear and 6" or 7" in the front.....but some people run 8" on both ends. You want that 8" rear rotor for safety, though. For street, people often run the opposite setup with a larger front rotor....and lots of people run 6" on both ends, too. It's just about preference. If you get into taller hookups or drops, be sure you have a strong dependable brake that is set up properly and burned in to make it grabby. You don't want your brakes to slip at the wrong time and put you off balance while you're up on a ledge.
The-Magic-Of-Making-Up-Review