I have a 6' blade that I used on my previous 21hp tractor and now use on a 30hp tractor. I would say stick with the 6' blade. It is not the HP of the tractor but the weight and traction that will come up short.
Not to argue with Jimmer2880 but I would think 12" of snow is way too much for even my tractor with the 6' blade unless it is nice fresh fluffy powder. I just plowed 10" of wet stuff with chains on all tires and the second pass was getting a bit too deep and heavy, but I got through it. I think a 7' blade would have stopped me on that one.
Not to argue with Jimmer2880 but I would think 12" of snow is way too much for even my tractor with the 6' blade unless it is nice fresh fluffy powder. I just plowed 10" of wet stuff with chains on all tires and the second pass was getting a bit too deep and heavy, but I got through it. I think a 7' blade would have stopped me on that one.