I just switched from a 6' to a 7' snow blower using a 50hp PTO tractor. Of all the things I do with my tractor, snow blowing requires the most HP. Using about a foot of snow as a baseline, I'd say the 6' is about the right size for 50hp. You need to go slow (walking speed) when cutting a full swath, and the engine will be bogging down, but it's fine.
I just switched to a 7' front blower, and my 50 hp PTO is under powered for it. So of the difference is due to losses in the hydraulic drive on the 7' blower vs a direct PTO drive on the 6', but 50hp struggles with 7', and I was only able to move about 1-2 mph cutting a full swath. An HST transmission is handy for regulating speed, but I'm sure a geared tractor with low enough gears would work fine too. It all works fine, but can be slow going in 12" snow.
In smaller downfalls, the wider swath is really nice because it makes faster work of everything, and you can still move at a reasonable speed.
Bottom line, snow blowing takes a lot of HP.