Taylor Park is fantastic. Grew up in CO and went there several times over the years camping. Brother just went up there in June.
One of the best times I had was just taking off on a trail bike and climbing up some really rocky trails all the way above timberline. Bikes are really nice as they can get on all the small trails, but there are tons of great places to get around to no matter where you go. Take a backback or fishing pole and go on a tour of all the trails and streams.
Have lots of fun either way. Not sure on the size limits. I can't recall any for sure, but i know ATVs and bikes are common. Can't recall any RTV type machines driving around, but may just be as they are more uncommon. Take a look at the CO parks site for the regs.