Setting the float to none is not always enough to get a 100% width. So I suggest adding a width: 100% to the size1of1 class. I personally ran into this problem when I was trying to make a textarea take the full width of the parent element.
If it can help with writing less code, why not? Why limit the way it can be used? All other size classes can be used to adjust the size of an element, if need be. Does adding a width property to size1of1 make it less useful in any way?
I know these size classes are originally meant to be used for making a grid and the layout of the page. But they can certainly help you Don't Repeat Yourself in other situations as well.