Since the ground loop is the biggest cost of having Geo installed it's critical to get this right for quickest payback time vs. a conventional HVAC system. Too small of a loop and you won't get the rated EER from the unit, too big and you overpay for installation costs. Reducing the installed cost (therefore payback time) is key to making geothermal a successful technology.
Does your calculator take into account the thermal mass of the ground?
The geo system may be designed for a peak load of 42,000 BTU/hr, but the average load may be closer to 480,000 BTU over a 24hr period (24,000btu/hr). The ground due to it's high thermal mass may only need to produce the average capacity instead of peak.
Does the calculator consider installed cost?
Installed cost varies wildly depending on site conditions and labor costs. Installing a larger loop might make sense at the time of construction since the digging equipment is already on site for construction of the building. There are no landscape considerations to worry about. Same may hold true on a rural farm where land is plentiful and labor costs may cost substantially less. In an urban area where installation costs are high, it might make more sense to size for cooling load and have backup heat strips pick up the rest on those really cold nights.