No, buying online isn't a bad idea. If you know your fit, can put a bike together yourself, and can adjust small things like brakes/derailluers, then buying online will get you a better deal than buying from a shop.
However, if you fit a 13.5"-14.5" frame, I'm not sure you'd fit on the 16" frame.
However, if you fit a 13.5"-14.5" frame, I'm not sure you'd fit on the 16" frame.