I went to my local motorcycle shop and tried on a lot of different helmets. They all fit differently and size differently. Ended up buying the best fit/feeling one... a Scorpion EXO 400 in XL. I'd suggest you go to a store and do the same.
I'd say go to a big dealer with many helmets and start putting them on. I'm a 2x seems like with all MFG's. But try on many! You may find the one you thought you wanted sucks, and just don't fit you right.
They are all different. Go to the manufacturers websites and read their sizing charts. You want the one that fits the tightest without pressure points that cause pain. You don't want to be able to slip your fingers up between your forehead and helmet. Grab the chin guard (full face) and pull down - you shouldn't be able to slide it over your eyes. Shake your head right and left - it should move with your head without slipping. Most importantly - as said above - try on a bunch of them and wear them around the shop for at least 15 minutes each. Use the time to check out the bikes on the floor, try on some jackets, or sit and text your best gal. I've always been a fan of Shoei so I ended up with the RF1100. It's wind-tunnel designed and extremely quiet. It also has ear holes molded inside for speakers - great for a communication device, once again for your best gal - or for GPS, etc. Even within the same manufacturers helmets are made different. Take the Shoei, the model that my RF1100 replaced (RF1000) was made for a round head while the new one fits the longer type.
In the end it comes down to comfort. Get the most comfortable one you can find so you don't have an excuse not to wear it.