A 13 toother is way to small for my blood in the trails. I say a 15 is minimum. I know a lot say that the 13 will "give you more torque", which is kinda BS. It won't, it will make your gears closer and somewhat lower, but you'll find in the trails, you'll be shifting like mad. I'm running a 15/41 (22" rear) right now and I can blast tight trails in 2nd and 3rd easy and keep on everyone's ass, if I'm not in front, while they're shifting back and forth, and then hit wide open and be gone in an instant.
You just have to try the sprockets and see how you like them. I guarantee if you ride a 13 tooth and then a 15 tooth, you'll keep the 15 tooth, plus the 15 tooth won't spin your tires as much.