I would expect it to need a 5" bit but it all depends on that particular pony and the bit you want. My horse (16.1) is usually a 5.5 but she's in a 5.75" NS verbindend, whereas a 5.5" mullen mouth hangs out each side like she's carrying a stick. Here's an article with some ways of measuring.
Bit size tends to vary a lot with type of horse, IME. my 15.2hh ID x Connemara took a 5.5" / 5.75" but my 16.2hh TB takes a 5" - the same bit as his friend used, who is a 14.1hh riding pony type!
Bits are measured true to size between the bit rings, but the way it sits in the mouth determines which size you need - a straight bar "comes up bigger" than say, a Neue Schule Verbindend which is very curved in the mouth.
If you're buying a new one I'd suggest using a bit bank and finding what size fits yours best, based on the suggestions of about 5"