what size tire do you guys use?

You can run 18's on the trails but will get hung up a lot more. I have 18's on mine but will be switching to 20's as soon as I can.
 
According to the manual, stock rear tire is 21x10-8

I have 22x11-8 and 22x12-8, ride mostly narrow hill and mountain trails. They work fine but seldom get past second gear.
 
i run 20's in the back, and 22's up front for added clearence here in the rocks
 
20in ft
18in rear for mx

21in ft
20 in rear for xc,stay away from heavy tires,best i have found are gbc race rex for the rear or itp xcr
 
currently have 21" up front and 22" in the rear for trail riding, mud lites are the ticket, aggressive tread yet strong and light weight.

I wouldn't go any smaller with the riding I do. When the fronts are worn out I'll go with 22s up front again.
 
i run 22's on 9 inch rims. i had 20/11/8's but kept beaching on rutted hills etc so i went to 22's and dropped to a 13 front sprocket size, i used Luni's gearing calculator and it works out the same gearing as i had before with a 14 tooth.