I got my chassis online and they have the size measurement that goes along with whatever size hockey boot you have. I got L cause my boot is 11. Also, I had to get the holes re-drilled.
I only play roller once or twice a week and it's beer league stuff anyways so I opted for the Mission Aluminum chassis instead of Magnesium. I throw on some outdoor wheels to skate around on asphalt as well and the chassis and boot combo feels good.