What is the best sized rims for dirt on an X?

Noob question here. I've always had dirt bikes with dirt bike sized tires and rims. Now with the X, I've been considering what to do for tires for my off road ventures. What are the reasons behind choices?

21/18 is probably BEST, but a much less expensive option that work well is use the 17" rear and buy a 21" front from Warp 9 or Wheeling.

I have a 19/17 street/ dirt road setup for my KTM and love them.