How do you calculate the right size furnace to maximize efficiency?
With a Manual J heat loss calculation. The number depends on your house's size, insulation, and air sealing, among other things. So you (or the installer) need to know those things first. You can do a rough, ballpark calculation yourself at http://loadcalc.net/ (no affiliation). It won't be 100% accurate, but it's a heck of a lot better than using antiquated rules of thumb. Dismiss anyone who tries to use rules of thumb or blindly recommends replacing equipment with something of identical size. They don't know that they don't know what they're doing.