tire size

[yamaha1470]

Hey I like riding MX and want to get new tires so i am probably going to get the 20 inch Turf Tamers in the rear. My question is what is a good front tire for MX that I could use with a 20 inch rear tire? Can I use a 20 inch front and 20 inch rear? Are the tamer fronts anygood? If i cant run 20's front and back what is a good front tire that is 21 or 22 inches big?

Thanks:macho