As long as it's a single vehicle (not a trailer), you can drive any size vehicle here as long as it doesn't have air brakes (well, the limit is 26,000 lbs, but they all have air brakes anyhow).
Farm plates do make a difference, but for me the 150 mile radius would be very restrictive.. there's hardly anything within 150 miles of me.
For what I'm doing, a 20 ft (maybe 22 ft) with a good pair of 7K axles and 14 ply tires really does the job quite well.. a 70hp tractor is possible, as well as most implements.. and it takes me 2 loads to haul all my calves and a few cows.