I raced at 39x18 this year on flat courses. I mostly left the singlespeed in the van if the course had anything that resembled a hill.
Now that CX season is over and my SSCX bike is in commuter mode, I'm using 39x16 but contemplating a switch to 45x18 or maybe even 47x18. Possibly after a summer riding that I'll be ready to bump my racing ratio up a bit.