Go to your LBS and try a few on.
Best, dare I say to try your local
Halfords as they will invariably have a wideish selction of makes & colours to choose from. Although many bash hats now have a 'click-to-fit' system, the actual fit wil depend on; the shape of your head / the internal design of the bash hat. My own thoughts on a fitted rear light are to invest in an attachable one as they are easily removed for dag change and repalcement when they eventually pack up - !
Important to consider that when the hat is on youre head correctly adjusted and strapped, there should be virtually no movement back-to-front or side-to-side. If it moves then, it'll certainly move if you have a fall. Not advisable - !