I noticed that there are two tire sizes for the Monster trucks....What tires are you supposed to run in the class. The fatty ones or the low-pro ones. Say for example proline stuff

Are you referring to Max size and 40 series? If so, it depends on the vehicle in question. Both are the correct size for racing, the 40 series have more tread width, the diameters are nearly the same, maybe the 40's also have less sidewall, but OD should be the same.
HTH
Rod

yeah the 40's are lower profile....but are they just for the truggy's or are they also allowed on monster trucks

both, I know TONS of racers that run 40's on their revos.
HTH
Rod

yeah alot of people run 40s on revos. im about to get some. they are completely legal. just get what you want really