I've been trying different tire sizes on our bikes (250EX, 300EX, and 400EX) and wanted to share my results. All the tires are ITP Holeshots and all are mounted on ITP T-9 rims (10X5 front/9X9 rear). The 9X9 rears are an inch wider than stock (tire gets a flatter wider profile) and have a greater offset, so you get aprox. a +4 wider rear (which increases stability)! We ride desert XC, with a mixture of everything (except not a lot of thick gooey mud).

Front tires: Standard 21X7-10, XC 22X7-10, and XCT 23X7-10. All three tires have the same tread pattern, but profile differently on the rim. The 21 has a flat and wide profile, where the 23 is rather rounded and narrower, with the 22 falling somewhere in between. Rear tires: Standard 20X11-9 and XCT 22X11-9. On the rim, the 22 is quite a bit wider than the 20.

300EX-With 21 front/20 rear the bike is REALLY quick and agile, and handles every bit as quick as the smaller and lighter Blaster. This is the hot setup for trail riding, but you give up ground clearance. For XC riding I choose the 22front/22 rear for increased clearance and a bit more stable ride.

250EX-The front end pushes (understeers) badly with the stock tires and it is difficult to slide the back end. The bike is a little tippy too. With 21 front/20 rear the bike is very agile, and both the tippiness (lower center of gravity) and the pushy steering improve, but don't go away completely. The power is a little more torquey because of (effectively) lowered gearing and the back end breaks free when needed. With the 22 front/22 rear combination, the pushiness and tippiness disapear almost completely!! The front is very solidly planted and now the back end breaks free almost too easily. This bike will be ridden by kids/beginners, so we will probably go with the tamer handling of the 22 inch tires, but these are heavier than stock, so this is a lot of tire for the little 250 to push.

400EX-Here my choice is the XCT's all the way around (23 front/22 rear). Smaller tires seem to handle kind of squirley. The added ground clearance is great, and even though you would expect the big front wheels to tame the steering down, it corners and carves turns great. I think the rounded profile of the big fronts gives them more bite.

Some advice-The tire size may not mean a whole lot once the tire is on a rim. The 2 different rear tires had different widths once mounted, even though both are nominally 11 inchs wide. The 3 different front tires, each had very different profiles once mounted.