Every helmet is shaped a little differently. Your best bet is to go to a motorcycle superstore and try on LOTS of different brands. Then go to your local store or online and get a better deal!
Actually, I see from your other posts that you are in Charlotte. There are a couple of Cyclegear stores in your area and they carry a wide range of helmets. I've been to a number of their stores in multiple states and they have always been awesome to me.
Things to keep in mind when trying on a full face helmet (my personal helmet of choice):
When on the helmet should fit snugly - the foam padding will compress with use/age so it should feel almost too tight initially. It should sit square on your head. If it tilts forward or back it doesn't fit properly. The cheek pads should touch your cheeks without being uncomfortable. There should be no gap between your temples and the brow pads.
Once you have the helmet on and the chin strap done up move the helmet around on your head side to side and up and down. Your skin should move with the helmet. Can you roll the helmet forward off your head? If so then it is too loose. Try a smaller size.
After you take off the helmet are there any sore spots on your head? Is there red spots on your forehead? If so the helmet is either too small or badly shaped for your head.
Once you find a helmet you like, wear it around the store for a while. It needs to be comfortable for relatively long periods of time.
If you have the money I advise getting a Snell endorsed helmet. These helmets have been tested to destruction and are high quality. However there is usually a significantly higher cost factor and only a full face helmet will meet Snell standards (3/4 and half helmets have no chin coverage so automatically fail Snell).
Lastly, if you go to a store to try on helmets and the staff do not seem knowledgeable or get upset or annoyed at you wanting to try lots of different helmets, go somewhere else. You only have one head and it's important to get the best/most comfortable gear you can afford to protect it. Good luck shopping!