How to properly size a residential hot water boiler and radiators John
White, Energy Marketing Service 

A properly designed and sized hydronic
heating system will insure your comfort and save many dollars on
installation and operating costs. If you are replacing only a boiler and leaving the existing baseboard or radiant floor piping in place, then a simple whole house (block load) calculation will determine the new boiler size. WHOLE HOUSE HEAT LOSS CALCULATION
This load calculation indicates 53,687 btuh are needed to heat the sample home. Therefore a boiler with an output of at least 53,687 btuh must be chosen. If you are replacing the entire system, boiler plus radiators or radiant flooring, then a roomby room calculation must be performed. This calculation will determine the correct amount of baseboard or floor piping to be installed in each room.
ROOMBYROOM CALCULATION
The illustration above indicates 6223 btuh are needed to heat the kitchen. If the baseboard manufacturer's specifies 580 btuh/ft., then 11 feet of baseboard will be needed in the kitchen (6223/580 = 10.7). Likewise, the living room will need 7500 btuh to keep it warm, therefore 13 feet of baseboard will be needed (7500/580 = 12.9). A heat loss/gain calculation is not rocket science. One can be preformed by anybody capable of calculating the square feet of a homes' walls, floors, ceilings and windows. Below is a link to an extremely affordable (only $39) HVAC load calculation program that will produce a heat loss calculation plus a heat gain calculation (sizes A/C) and operating cost estimator. You will be able to compare one system to another or one fuel vs another. Its well worth looking into.
http://hvacloadcalculator.com/loadcalculator.html
