Sizing depends on a lot of factors. What is the insulation factor in your house, how many air changes per hour is your house experiencing (do you have a heat recovery ventilator) how tight are your windows and doors. What is your exposure like. Does your house sit in the middle of an open field, or is it surrounded by a good wind break.
Many guys will just replace with the size of unit you already have. Others will use rule of thumb calculations. Even others may just pick the closest size that thier particular manufacturer makes in the equipment models they are trying to sell you. Most high efficient models come in 40, 60, 80, 100,000 btu's. If you currently have a 70,000 mid effficient, which would they recommend?
As far as run times go, you would expect your furnace to run fairly constant on the coldest days. Generally you would expect that on the coldest day you would ever expect to see in your area, you should expect to see your furnace run 85-95% of the time if it is sized close to correctly.