One of the biggest hurdles to keeping kids rearfacing is the parent's belief that their legs will be cramped as they get taller and that they are uncomfortable. During my CPST class last week, some of the other people in the class were questioning how a child would fit. So I took some photos of Lilypad Princess in her seat to show exactly how she fits.
LP is 2 years, 9 months old, 27 pounds, and between 36-38 inches tall (I know that she fit on the rides that required she be 35 inches tall but was too short for the 40 inch rides at Disney). She has consistently been between the 95th and 100th percentile for height.
How she normally keeps her legs
How she stretches her legs
She never complains about being in her seat, and usually falls asleep if we're in the car more than 15 minutes, so she is obviously quite comfortable. We have done 4 hour trips without any complaint. When she has been forward facing (in her travel seat) she has actually complained because she didn't have anywhere to rest her legs and they hurt just hanging down the seat.
She will stay rearfacing in both of our cars until the limits of the seat, 33 pounds for each of them. At the rate she's going, I expect her to be 3.5 or almost 4 by the time she reaches that point (unless she outgrows the seats by height first!)