Apparently height at two years is approximately half the expected adult height.
Siobhan Pittock, M.D., Pediatric Endocrinology, of the mayo clinic says this on the matter:
A child’s height at age 2 may be predictive of final adult height. But many factors can affect height…
Doubling a child’s height at age 2 can provide an estimate of how tall that child will be in adulthood…
After age 2, most healthy children grow steadily at about 2 inches per year until they reach puberty. …
Maria Scinto at Livestrong offers this:
… Adult height is influenced by various factors, including genetics, nutrition and overall health — but even having the same genes and eating a similar diet while growing up do not mean two children will grow to be the same size…
the simplest way of predicting adult height is by doubling the height a boy reaches by age 2. Girls develop more quickly, so you can double their height at 18 months to come up with an estimate of how tall they will be as adults.