I took mine in to a local reprographer who could enlarge or reduce out to two decimal places. Accurate sizing means reducing the WOF templates to 98.77% of their normal printed size (on regular letter-size paper, not A4) for accurate Boba size (although the details still need to be adjusted -- Alan's working on that update as he can). To make it Jango-sized, it needs to be reduced further to 99.1% of the Boba size.
Many printers I've run across don't graduate by more than whole percentages, one decimal place if your're lucky.
These are considered to be the two primary benchmarks. So if even a Jango helmet would be too big on you (and remember, you're looking for something that looks right when you're in armour, not normal street clothes), try... reducing ten percent? Try 90% of full size. The helmet is often "oversized" on one, but hard hat suspension or good padding will solve that. What matters most is what looks right on you when you're in the rest fo the costume.
--Jonah