There are NO quick-n-easy methods to calculate these factors. The easiest is "Right-J", distributed by ACCA _Air Conditioning Contractors of America), it's a software program where you input the type of construction (brick, wood etc) along with numerous other factors to achieve the Heat Load/ Loss of the building.
It takes years of training and experience to be able to perform a competent evaluation of the heating/cooling requirements for a structure, and have the system perform as designed.
Sorry, there are NO easy fixes. I know, I am a licensed Master HVAC/Gasfitter in Baltimore City. We are often contacted only AFTER the homeowner makes a failed attempt at installing a heating system.
Fortunately you asked before you leaped. Contact several contractors in your area for estimates, then select from among them.
I am sure the results will be satisfactory, and the installed system will perform to your expectations.