I love using OOCSS grid names like .size1of2 or .size3of5, they are really handy. The problem I have with them though is that they make no sense in a responsive design where a grid of .size1of3 on a big screen might be a grid of .size1of2 on a slightly smaller screen.
Use the same classNames and don't care about the exact width. So a .size1of3 can actually be 33% wide on a big screen and 50% wide on a smaller screen and probably 100% on a very small screen. I have a problem with this, you might understand that.