my 2 cents and NO I don't know the formula.
is a simple way is go to a good plow brand's site , type in your trucks info and it will recommend the plows that will fit for you.
NEXT if you want to go BIGGER, there are ways you can set your truck up to handle bigger, first will be air bags to help HOLD the load, at some point you will be in need of some ballast to counter the weight of the plow
bigger also will be more wear and tear on the whole truck, more front end, but whole truck .
I personally wouldn't go larger than a 9 ft blade and even that can be hard on a truck, pending design of it and weights
is your truck a GAS< or a diesel?
as that factors into the big picture on weights
here is a decent read for you, has some sort of a formula in it as well, but just good info , have a look
http://info.bossplow.com/Blog/bid/153972/Before-You-Buy-That-Diesel-Snowplow-Truck