We are on our second winter with ours.  2400 square feet plus full basement.  We have a 7 ton unit with 8 loops run in marshy ground below house.  Last winter we had about 8 days that the temp stayed below 0 with most night running -20 to -25.   That week was the only time our back up system with LP ran at all.  The only reason it ran was because we are on dual fuel with the electric system and they can shut us off during peak demands for up to two hours.  We were getting shut off for two hours in the morning and again at night.  The LP kicked in for one or two cycles each time the electric was shut off to the primary system.  When the weather was not that cold the heat for the in floor loops in the basement floor was enough to carry us until the power came back on.  I think we used less than 50 gallons of LP during the whole winter.  If we did not have the dual fuel I don't think the geo would have any problems keeping up.

When we built we spent extra on insulation and windows and other things to make the house pass energy star.  When they tested it they check for air leaks and we needed less then 2000 scfm air loss and we had 700.