The helmet MUST fit correctly to protect. You'll be setting yourself up for liability if you try the baseball cap thing. Would you really want that on your conscience if your passenger were seriously injured because of a poorly fitting "one-size-fits-all" helmet? What if it was your girlfriend/sister/cousin who ended up with brain damage, and YOU were uninjured?so how about for a passenger helmet ?
the lady at the store told me that most people come in and grab a medium, and then ifyour with your lady mamma (hah sorry ive been listening to steppenwolf latley) and her head is too small she can wear a baseball cap or something underneath.
obviously if your gonna have someone specific riding on the back you should get a proper helmet for them, but if this is a general passenger helmet what are your thoughts ?
Wait until you've got someone who you expect to regularly take pillion, then buy a helmet that fits THEM. It is just NOT worth the risk.