We have an existing furnace with an AC coil but think the condenser is too small for the square footage of our house. Can we just use the coil in the furnace without switching it out with a larger one and just hook up the larger condenser?

Actually they are pretty much matched up to the capacity of the outside unit. And with A/C the condeners is the outside unit, the evaporator is in the furnace...

The Evaporator absorbes heat via expansion of the refridgerant (i.e. It gets cold)... and the condeser literally re-condenses the refidgerant and in the process releases heat (I.E. it gets hot). The Roles can be reversed with Heat Pumps.