Make sure you are not confused by the 16" size given - that is the wheel size, not the bike size. In general, you can expect folding bike sizes to be the equivalent of a 20" triangle frame bike.
If you are after a bike for a small adult, it might be beneficial to look at racing bikes for high school children - these are serious bikes but obviously designed for small people.
Bike shops is also a good place to ask, as they would get requests like that from time to time.