depends what/where you ride.
if you like spinning alot, go with 18"
if you need some clearance but want to be able to spin the tires, go 20"
if you want more speed and lots of clearance, go 22" (may need extended swinger)
also the type of material you ride on chooses your tire. dont think "holeshots" because its a popular name
(we can go a lot more in depth if you let us know what you ride on)