Snug, so long as there are no pressure points, is what you want. I have a very large, very hard to fit head, and I get extreme pressure across the forehead in a LOT of helmets.
If the helmet is just a snug, cozy fit all the way around I'd call it the right size.
I managed to try on a Shoei Hornet DS on our recent trip South. It fit like no other helmet ever has before
but I didn't buy one.... That price!
Yes, I've paid that much for a helmet before, and am sure I will again, but just couldn't commit to it that day...