My melon measures 24 1/8". The helmet is just a tad bit round for my head shape. My brow hits the front and it feels a tad loose around the sides. HJC helmets are pretty true to size. If anything, they run a little on the large side.
These are the helmets I have by comparison:
THH Skull Cap (XXL) - A little loose
Scorpion EXO 1000 (XXL) - A little tight
Nolan N103 (XXL) - A little tight
HJC FS-16 (XXL) - Fits perfect
Scorpion EXO 100 (XXL) - Fits perfect
Hope that helps...