Many ask if there is a tonnage to sf rule of thumb, afraid that if equipment gets too small they will sail off the edge of the earth. Kinda like happened to explorers up until Columbus magically reshaped the earth from a pancake to a globe.
That reshaping is occurring now in our industry. (of course stupid setback strategy thing needs to be weeded out of the public consciousness to really succeed).
Well, there are some using this rule of thumb in reverse. If your equipment is more than 1 ton per 1000 square feet, the HERS guys suspect crappy work and pull out the magnifiers.
Here's a GREAT article on the subject...