I would definitely go for a 160 Hope Mono trial, they've got sufficient power, I'm even using one on my 26" and have been for nearly a year now. As long as you keep the pads and rotor really clean then you shouldn't have any problems. Other disk brakes include the Magura Louise, which is alright, just not as powerful as the Hope from my experience. The other route to go down is an Avid BB7 cable disk, they work very well, but I prefer the feel of hydraulics. Edit: Oh, and for the rear, I would recommend the same size for 20" wheels , just keep them free from grease and grime.