As BayAreaCool aluded to, unless you are just looking for confirmation that a vendor is quoting you a reasonable sized unit, it appears you may be going about this selection process backwards.
Direct answer - depends on where you live. Unit has a usable heating output of about 90,000 Btu at high setting. However, depending on where you live in the US, you could need from 10 Btu/SF in Orlando to as much as the 300 Btu/SF I have seen in poorly insulated houses in Barrow, Alaska - so by very rough numbers, without the benefit of actual energy demand analysis, could heat from a 300 SF quonset hut in Barrow to a 9000 SF home in Florida - pretty much covers it all, doesn't it. And demonstrates how, without proper data on your specific locale and house construction, you can pull a flat number like 400 or 500 Btu/SF off the web and get a result that reflects the age old adage - garbage in, garbage out.
First, you should decide if you are going to do insulation, weatherstripping, etc as part of your upgrade - this can affect heating load calcs. You don't have to have them done yet, just account for if going to do them for certain before the next peak heating period, though certainly best to get them done first if possible (if this is a planned rather than emergency replacement), then get an insulation audit and blower door test done so you have "real" numbers regarding natural air exchanges, air demand and house permeability, etc.
Second - get am ACCA Manual J calc done - this determines the heating "load" or heating energy demand of your building.
Third - a Manual S calc, which determines the unit sizing and heating output required.
Fourth - a Manual D design of the ducting required
Fifth - a Manual T analysis of the Air Distribution components - grills/grates, dampers, etc
Here is a link to what I think is a very good, non-technical explanation of the various steps needed in sizing and designing a forced air system, and one of the few that emphasizes the importance of Manuals D and T also.