I know this is an old thread bump, but certainly applicable for anyone searching for this advice, albeit 7 years later, ha!

I recently picked up an MRR2 and have been begun evaluating it here:
http://www.ultimaterc.com/forums/sho....php?p=2232333

The manual doesn't specify what size servo it needs, however I found that a standard servo won't fit with the pre-drilled holes in the chassis. My kit didn't include any servo mounts either. I found that a mini servo will fit nicely:
http://www.hobbyking.com/hobbyking/s...05sec_34g.html



Only problem is that the servo mounts I'm using in that pic are 7mm thick (threaded holes 3.5mm on center), and I'm in the process of custom building 6.5mm mounts with holes 3mm on center from the wall of the servo to get a perfect fit. I'll be updating the review thread later this week after I finish milling custom servo mounts with my CNC.