REARFACING seats are outgrown by height when there is 1" or less of hard shell left above the baby's head. The exception is if there is a strict height limit--both the Cosco Scenera and the Safety 1st Complete Air65 have them. (Always check your manual to be sure.)
FORWARD FACING seats are outgrown by height when one of two things happens. 1) their shoulders are above the top harness slots; or 2) the tips of their ears are above the shell.