I tried a FX-37 and really liked the look and the price...but it was tight front to back with a lot of room on the sides. It was definitely designed for someone with a perfectly round head based on the shape of the foam under the liner. It also had minimal padding at the forehead. It was still a great helmet for the price, but it just didn't fit so I had to send it back...which was a piece of cake with DK.
The FX-39 looks even better, but I'd bet the foam shape is the same. If the 37 fits you then the 39 probably will too. Good luck!
08 Suzuki DR 650SE
99 Suzuki DR350SE
07 Buell XB12X