15/44 (stock S gearing) would be a good start. Keep in mind that a 18" rim with a knobbie on it is larger in diameter then a 17" rim with a street tire on it. So if you were to use the same gearing it would be even taller( and less suitible for dirt use) with the dirt rim.
15/44 works with the same chain on my SM. You could go 14/44 and that would work even better for the dirt in most areas, but that would mean dealing with swapping the counter shaft spocket.
14/44 would be better, or you can even go 13 up front. It all depends on the type of dirt work you plan on doing. If you are talking fire roads and open area, the taller gearing will be ok. If you are talking about tight technical stuff, gear it down as low as you can.
I run 15/39 in SM mode, but when I put the dirt rims on I run 14/47 or 13/47 depending on where I plan on going.
i'm running 15/44 with a dirt set-up and 15/41 sm with the same chain... i'd like to go bigger off-road but if your gonna keep the same chain to keep wheel swap time minimal then you gotta compromise somewhere... you can run 15/44 - 15/47 with a couple more links but i don't really like 44 on the street