"guest" helmet sizing?

[mattsz]

I know that there is no substitute for a properly fitted helmet, but at what point is "close" no longer better than "none"? 

I would like to get an extra helmet to have on hand for when somebody wants to ride on my bike with me, and I can't see carrying a collection of every size helmet around with me just in case.  I know too small might be uncomfortable, but I'm not talking about all-day rides, just quick jaunts.  I also know too big can fly right off in a pinch and be useless...

Is there an average size that would be most likely to fit any random person trying one on?  What do you guys/gals do?

[Chuck D]

Medium.

[Rich Mintz]

I bought a small (half-head) XL to carry with me (bungeed to the rear seat or clipped to my saddlebag) on trips when I might have to carry an impromptu passenger a short distance. XL because most people I know have large heads.

There happens to be a spare full-face L helmet at my work (abandoned by some previous employee) that's at my disposal.

[benknrobbers]

a medium 3/4 face seems to work really well. You don't have to squeeze your head through the bottom hole, so people who are cranium challenged seem to be able to manage it without mush difficulty.