Correct, the size is the size of the rotor. The bigger the rotor the more stopping power, but as the rotor gets bigger the more likely that it will get bent out of place which could cause annoying rubbing issues and more maintenance to bend back in place. For the Midwest you should be fine with 160mm, but since the 180mm is cheaper you might as well get the added stopping power for the price. Just note that if you do buy the 180 and anything happens to the rotor then it looks like it's hard to replace it with another Avid rotor (non standard size for them I guess). Remember that these are sold individually so you'll have to order two to get front and rear.