I have run a 72" plow on my 500 sportsman for about 5 years with no problems.
I get a lot of heavy wet snow.

 

That will do it. A 2wd with chains has more traction on snow then a 4wd with no chains,I would think.

 

I would imagine as long as you keep up with the big storms you wouldnt have a problem. But, last year i was away one night and we got a huge storm...about 18" of the heavy stuff. Not fluff. Took me 3 hours to plow out my drive which usually takes 1 hour. It was not easy and was harder on my bike than i like to be. Pretty much had it pinned for the whole first hour trying to not get stuck.

Now usually i would plow a couple times during a storm like that and have no problems doing 4-6" at a time. That storm rocked...i just wish i would have been home to enjoy it! I couldnt even get my truck in the drive when i got home!

 

I had a 60 inch on my 700, I was always going to add 12 inch wings to the side but never got around to it. The 60 just felt so small, I know I could have handled a lot more before I started taxing the machine.

 

Im quite happy with my 60...it feels like the perfect size for me...maybe we are doing 2 different things, and maybe because i went from a 54 to a 60. Around my house and garage i feel like anything bigger is going to be cumbersome to get up close. Also, on the real wet heavy stuff with my blade angled i will literally have it pinned going out my drive (its about 1/4 mile) and my bike will be sideways half the time because of the angle of the blade. But if you go any slower you will just sit there and spin. I can clear my drive in one pass up and back and then on the big storms i take a couple extra passes and push back the banks. I just have no desire for a bigger plow...plus my front springs are stock and i dont want more weight up there unless i get stiffer springs. Now, if i were doing flat parking lots or something i might feel differently. i have 3 years on the 60 and i had 3 years on the 54...i didnt like the 54 at all because it just wasnt wide enough at full angle.

I am thinking of adding chains and stiff springs this year. I also added a piece of rubber belting to the top of the blade and thats the best thing i did yet! No more snow in the face.

 

I would deffinately have a hard time pushing 18" of snow. I like to get out and move it with no more then a foot.
I really don't think it's a matter of HP. It's traction. I'm usually not much over an idle while plowing and raising the blade at the end of where I'm pushing helps reduce stress on the plow frame. Heavy front shocks are a must, the stockers bottomed right away. I do weight the rear with a back blade which also helps a ton by not needing to change directions so much.
Where I do have a problem as trying to clear my daughters city sidewalk. The front plow is just too wide. I can use the back blade but it's not the best set up.
I have had to raise the plow and take a light pass in heavy snow them go back and clean up.
I had some pictures of my plow settup but have lost track of them when they changed the site awhile ago. Are picture that were there still posted on the sit someplace?

 

Yeah, the 18" was bad. I was HARD on it. i basically didnt even lower the blade and just mashed on it and just kept taking layers off till i got to a point that i could lower and scrape like normal. It was really my only option. I couldnt get a plow truck up the hill at the bottom of my drive. Once i had it plowed out my bro in law came with his truck and took the banks back because my drive ended up not much wider than my 60" plow by the time i was done!

 

Screw it, Im gonna go with the 72" Moose system, I'll post pictures and let you all know how it works come Snow season!

 

Go Big or stay home.
I'd get a set of heavier struts up front, I put "Hilifter" on mine. Stopped if from sagging but still has a decent ride when the plow is off.