I own a 6 unit 3 story brick apartment building in Nebraska. The building and furnaces are 40 year old. Each unit is 950 sqft, 5 rooms (2B, 1B, K & LR), has appx. 100sqft of single pain windows. One contractor wants to sell me a 75,000 BTU Goodman the other a 50,000 BTU Comfortmaker. What size is the right size.

I believe as a guss the 50,000 will be plenty but you should have a manual J done to know for sure. A large or to small furnace can cause problems.