How large should a bike helmet be?

ALL Of it!! A helmet should fit you snug, not tight, not loose, just right. Avoid the ones shaped like footballs. They really dont protect much. I prefer one that covers the whole back of your head and your forhead. There the ones skateboarders wear. I have them for my boys. They have fallen many times but never hurt there heads.

Thanks Ben another response worth posting I wonot tell if you dont tell??
To be effective as head protection, it should be worn no more than 1 finger-width above your eyebrows (wearing a helmet cocked to the back negates most of its protection). Forehead coverage is more important as most bicycle crashes are "faceplants" but back of the head protection is a bonus that more expensive helmets provide.

Helmet makers specify a range of sizes for their helmets that is based on head circumference. Once you determine the proper size, check the fit and comfort. Fit should be snug. Sizing pads are usually provided. Most importantly, try to wear the helmet to see if your head shape is compatible with their helmet shape so you avoid getting one with pressure hotspots.