6 foot. I have a Deere 4410. It came with a 84" and all id did was push the rear end of the tractor around. It depends of if you have weights as well. I had my AG tires filled and it would still push the tractor around.
I think it also depends on the shape of the blade. I know the one pictured above has the flat blade that is bent. This is the way some are made because it is cheaper and was how my 84" was. My landpride has a curved round blade and the snow rolls off it like a wave. Way better design and requires less power to push the snow and will throw it off to the side. That is based on my experience using both on the same tractor.
The other note, is make sure you can rotate the blade 180deg while on the tractor. There will be times you want to rotate the blade 180 and drive backwards. There have been other recent posts on here about not being able to do that with some bladed while on the tractor.
Is there any truth to curved vs flat and bent as to performance?