correct phrase

either one is ok

but the first one sounds like it's for a really short boy, so I'd go with the 2nd one

A boy's 16 inch bike is the correct usage.

Using a "16 inch boy's bike" could lead the reader to think the boy was 16 inches tall. (Unlikely I know but thats the way it reads)

Thanks, Happycat.

Thanks, Dadpad.