The rule of thumb is a foot wide per 5 horsepower, but that isn't hard and fast. Your tractor is 18 horsepower and 15 pto horsepower. My B7100 is 16 and 13, but I run a 4' tiller and a 5' finish mower, no problems. Hopefully, someone on here will pipe up that has the same tractor, but if I had to guess, I'd say you are real safe running a 4' tiller and a 5' bushhog, and you may be able to go a little bigger on the tiller. I have no idea about a harrow.