Not so much the hp making the difference but more of the weight of the machine. My bucket is 5' so of course makes a 5' "pusher". I have chains and weighted tires. It'll walk a 25" dump. 8"-10" of wet snow is the problem. That stuff just packs and you're pushing hundreds if not thousands of pounds in front of the tractor. I concluded that I'd rather keep moving than to go back and forth with a wider pusher. I needed something "all-around" capable as we can get any kind of snow in this state. For my 4000 lb machine, 5' fits the bill for all kinds of snow.
TomTint is spot on. It is certainly the character of the types of snow in your domain that tells you how wide to go. If you're the type to get out at 11 pm while there is 4-6" on the ground and you wish to stay ahead of it, then your pusher can be wider. Me..I like to wait out the storm till the next day and deal with the entire mess all at once.