Fish,

how long does the furnace you have now run on the coldest days to wamr
the house...

if it has to run only about 50% of the time on the coldest days then
you are probably ok getting a smaller one...

if your present furnace has to run 100% of the time on the coldest days
to keep the house warm, then a smaller unit may not deliver enough heat
on the coldest days...

just figure a furnace with 1/2 the output will run about twice as long
as your present unit...

Mark