For the price of fuel, which can be bought for farmcash and regenerates after time, it plows 2x2 squares. The size of the tank is fairly small, and an alternate way to plow is to block in your avatar using fencing, hay bales, or animals that have been commanded to stay, and then plow the land that way, because you don't have to wait for your avatar to move. This also works for seeding and harvesting.
however, your fuel allowance stays the same no matter how many tractors, seeders or harvesters you have. effective use of them will almost certainly require you to purchase fuel (w/fv cash, not coins).