With the statistic of over 66% of installations being sized up wrong, how do you find a reputable installer for Geothermal?

Did you see this number posted somewhere?

I would get multiple bids,  find out how long they have been in business, ask for a list of previous customers,  check with the customers and verify everything the contractor tells you. 

Then ask them to project your actual costs and see what kind of guarantee they will give.  Withold part of the payment until you can verify that the system is working properly.

Good  luck and let us know