Aside from the motorcycle itself, many would argue that a helmet is the most important piece of equipment a motorcyclist can have. It controls the amount of noise you hear, the temperature of your face, vision, and of course impact and abrasion protection in the case of an accident. If you are reading through this with any doubt in your mind that your helmet absolutely rules, going and trying on a few different options couldn't hurt.
TL; DR: If you're not confident in your current helmet, go to a store and try on some of the major brands to see what's out there and what fits. Helmets should be nice, not a burden.
For my first year or so on two wheels I wore a simple Fulmer open face helmet. I was sixteen and mainly riding smaller bikes and scooters, so this was perfectly adequate for the low-speed, round ton putting I was doing. When I stepped up to my first "real motorcycle," I was given an old car racing helmet that looked cooler than hell, but was ill fitting, bulky, loud, and not too easy to see out of. Because I really didn't have anything to compare it to, I was stoked!
Looking back, I had no idea how unsafe I was being. I was more concerned with fulfilling the legal requirements than protecting my dome. Now riding to work everyday in a perfect fitting helmet with a few little modern touches, I'd love to help keep some one from making the same mistakes I did.
Companies like Shoei and Bell Helmets offer custom fittings at certain trade shows and events but for 99% of us, it's going to be a huge task to get out there and have your head measured. For the average Joe, chances are you've got a Cycle Gear or a motorcycle shop somewhere in your vicinity that sells a range of helmets. If dealerships are all that you have in your area, I would recommend going somewhere other than an H-D dealership, as they tend to have less brands and full-face options. Go in there and try on everything you can get your hands on. Find out what fits your weird head. You don't have to have the money to buy the helmet right there, heck you can even go back online months later and buy the one that you now know fits.