Sizing has gone all over the place in the last few years, and HJC drastically changed their internal shapes to be more round about three years ago. Bastards.
There are some great new brands on the market, but their sizing is completely whacked -- even their XXL and XXXL helmets are about the same as a normal L. (Icon, LS2, Nexx). I was severely disappointed to find that the Icon Airmada (touted as having a long oval shape) in XXXL was smaller
than the HJC XL I was wearing. It's a fantastic
helmet -- I wanted it so bad -- but it doesn't come in men's sizes.
You might have to spend some dough -- as others have noted, step over to the gold-plated Shoei or Arai aisle and you'll probably find that which you seek. Limber up your Visa, though...
You might also check with Bell -- some of their new helmets are quite nice, and available in larger sizes. No idea whether they even make a flip-up.
If you can't spend the long green, look into AFX helmets -- they make a line of helmets specifically in larger sizes for those of us with abnormally large brains. If you're into the dual-sport thing, the FX-39 runs rather large, and seems to be more oval than many.
Again, no idea whether AFX sells a flip lid -- I'm a full-face sort of person for very good reasons, but that's another debate for another day.