Hazmat is right. The rolling diameter of the tires must match, if they don't ugly things happen to the drive train. It puts a lot of strain on all the drive parts, besides that the tractor will not want to move if the size is off.
I found this out the hard way on an old 1947 Jeep CJ2A I once had. I got a good deal on two tires which were about 2 or 3 inches in diameter bigger than what was on the Jeep. So I mounted them on the rear wheels. When I put the Jeep in 4WD the front tires left skid marks in the dirt as the rear wheels pushed them along.
To make matters worse I could not get the Jeep out of 4WD, too much wind-up in the drive train. I had to jack the front of the Jeep off the ground before I could shift the transfer case.
Randy