It's been a couple of months now, I *think* it is 1/2" but don't quote me on that.

I just brought my old breather hose to the auto parts store and sized it up w/ the cut-your-own hoses. It was a little bit of a tight fit because the inside diameter was less even though the hose was technically the same size based on outside diameter, but with a little help hose got on just fine...