As a rule of thumb, you will need approx. 30 BTUs of heat output per 1 sq. ft. of space. So, a 40x60x14 building will require around 126,000 BTUs and a 40x60x16 building will require around 144,000 BTUs.
Factors such as local climate, insulation, and the interior height of the structure must be considered.
Calculate your EnrgyStar savings here.