You've got to try them on before you buy, I can't wear a lot of lids because I have a wierd shaped head and quite a few of the spesh/giro ones put pressure on a sticky out bit i've got at the back.
As a general rule, the lid should fit comfortably on your head without the chin strap done up. Put it on, tighten the retention system at the back and wobble your head around a bit to ensure it sits well. Even put your head down towards your knees as a final test. The lid should sit above your eyebrows at the very lowest and ideally a bit higher. The chin strap is very much a lats resort to keep the helmet on in the event of a crash and they can be almost infinitely adjusted, so don't worry too much about getting the strap uber comfy first time.