I am a 5'2" 22/24 and fit fine. I would think the being tall may be more of an issue. I am usually the last one out of the car at the end of the ride (I am not very "graceful" in getting out of the car). I usally ride with my DS (11).
There are test seats, but you have to ask for directions. They're no longer out front - you go in through the exit, to where the ride unloads, then down another short hallway and through a door, all for privacy, I assume. I never did try the seats because I couldn't climb into the car (broken hip, healing but I can't raise my leg more than a few inches)
But when I told the CM this, she said they had a car with a door that opens! So I waited, and sure enough - I could step right into the limo and pull the harness down safely over my I'm not sure what size but VERY fluffy torso! You should have absolutely no trouble, and a blast!
Im a size 20/22 but was a size 26 when I went last year and I fit in everything and Im almost 6 feet tall. Rock N Roller coaster is great. Getting out if a bit of a trick I just steped on the seat and heaved up. All was good.
I'm 5' 9" (tall) and also 22/24 and I had NO problem on any ride at WDW except for the spinning rollercoaster in Dinoland at AK. That ride is a tight fit if you are sitting next to another adult, but other than that, I had no problems on anything. I rode RNR eight times! It is an AWESOME ride!