How do you size your motorcycle helmet?

I just bought an AGV K1, It was so tight at first so I knew I had to break it in, now that ive done so, when I shake my head there is the smallest amount of movement, I measured my head and it's like 56 to 56.5cm which would be a small. I bought a medium. Is this 1cm to .5cm a huge deal? I just don't want to compromise my safety and I'm curious to hear others input on their helmet and sizing recommendations. I remember trying to fit a small dirtbike helmet about 2 years ago and I got such a bad headache I couldn't even break it in.