There are loads of suggestions online for how to determine the requirements by room for electrical baseboard heating units. I've condensed them as follows:
1) For an Old/Uninsulated home: Use 12.5 Watts per square foot of floor space
2) For a typical pre 1980 home with some insulation: Use 10 Watts per square foot of floor space
3) For a post 1980 home with decent insulation: Use 8 Watts per square foot of floor space
4) For a modern highly insulated post 2000 home: Use 6 Watts per square foot of floor space
Since 1 Watt = 3.412 BTU's, it seems that this rule of thumb guide should be capable of being extended to calculate an entire homes heat demand in "output" BTU's.
Example one: A 1,500 square foot non-insulated older home
Use category 1 above. 1,500 x 12.5 x 3.412 = ~64,000 output BTU's required (an old 65% efficient 100,000 BTU furnace should work well here)
Example two: A 3,500 square foot post 1980 home
Use category 3 above. 3,500 x 8 = 3.412 = ~96,000 output BTU's required (a 96% efficient condensing NG or Propane furnace rated at 100,000 BTU's should work well here)
Example three: A 2,500 square foot 1970 home with some insulation
Use category 2 above. 2,500 x 10 x 3.412 = ~85,000 output BTU's required (a modern non-condensing 85% efficient 100,000 BTU furnace (NG, propane, or oil) should work well here)
Example four: A 4,650 square foot modern and extremely well insulated 2011 home
Use category 4 above. 4,650 x 6 x 3.412 = ~95,000 output BTU's required. (a 95% efficient condensing furnace rated at 100,000 BTU's should work well here)
In all 4 examples, a 100,000 BTU rated furnace (of one form or another) should suffice. It shows the overall variability of heat demand to be huge. It's also one thing to get it wrong for a single room, and another thing to get it wrong for an entire house.