I've tried them. I tried loads on when I bought a full face last year. My LBS (Leisure lakes) were really helpful in ordering helmets in and keeping them as stock if they didn't fit.
I tried helmets from Giro, Troy lee, Urge and Fox. If I remember the Urge only comes in small/medium or medium/large, with pads to fine tune the fit. I think the medium Giro was too small, and the large was too big, I had the same problem with the Urge so their sizing is probably similar. The Troy lee felt the best, possibly down to the much larger range of sizes, but in the end I couldn't justify the cost. I went with a fox in the end as it also fitted well, just a bit heavier.
You can't adjust a full face the same as you can a standard helmet with the retention straps, it either fits the shape of your head or it doesn't. I thought the large giro fitted me but I was told to shake my head around without doing the chin straps up and it flopped back - which is potentially worse than no helmet at all.
My point would be that you really need to try before you buy, even if it means a long trip to a decent shop.
Some guys they just give up living/And start dying little by little, piece by piece/Some guys come home from work and wash up/And go racin' in the street.