Hello folks, we are hoping for any comments to help us better determine appropriately sized units for our 1965 house.

2800 square feet, single story, long narrow footprint (long exposures are south/north.) Stucco exterior, low pitch shingle roof, well insulated attic. Windows have been upgraded to double pane, but we still have 44 linear feet of old single pane sliding glass doors, so the house can't be considered "tight." Three small skylights, one wood burning fireplace, mostly 8' ceilings, two people. We are in southern New Mexico, elevation 3800.

Current split system is two evaporative coolers, and two old gas furnaces, 60,000 and 75,000 BTU. Smaller furnace serves the smaller (bedroom) 1200 square foot part of the house. Larger furnace serves the larger (kitchen, LR, dining, den) 1600 square feet part. This system works pretty well except for a few rooms on the larger end that don't seem to get enough heat/cooling, but we're not sure if that's due to a duct work issue on that end, or current unit size.

We are upgrading to two packaged rooftop units, leaning towards Trane, and have bids from four local contractors.

Some contractors suggest Trane XL14C (2 stage 14 SEER) 2 and 3 ton units, 64,000 and 75,000 BTU. Others recommend Trane XL14C (2 stage 14 SEER) 3 and 4 tons units, 75,000 and 96,000 BTU.

We're focused on the variation in recommended size. We've read comments here about load calculations, but perhaps this is a more common practice in some regions (?) as none of these guys have mentioned load calcs at all.

We're mostly concerned about getting the heating component too large, and having problems with short cycling and wear on the system. Our contractors don't seem as capable of explaining how they arrive at their size recommendations as many posters here; they seem to go by experience, which is fine, but it's harder for us to determine which size is the best bet.

Is this enough info to gather a few comments about unit size? Thanks for any thoughts.