Your first step is to determine whether you need gas or electric. If you are in a northern climate and gas is available this should be your first choice. To determine your gas furnaces' capacity, remove the service panel from your existing furnace and look for the Btu rating; either the "input" or "output" rating. 

Gas furnaces are sold in 20,000-25,000 Btu increments. You don't have to nail your heating requirement down to the last Btu; just get as close as possible then select the furnace with the nearest capacity, rounding up.