also for atv tires read 20x10-8
for mx/well groomed trails a 18" tire would do great.
for the normal trails a 20" is the best option
for anything that is rutted and very rough, a 22" tire would work the best.
dont forget that the 18's and 22's are going to alter your gear ratios, the bigger the tire diameter, the loger the ratios will become. well the ratios dont really change, rather your final output speed
^^^ true. a taller tire will give higher top speed and a smaller will give better acceleration.
you can help get it back to stock by changing the sprockets.