This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The sizes of pipes should be governed by the amount of radiating surface to be supplied, the height of the radiators above the boiler, and the number of changes in the direction of the several currents. It is considered safe practice to allow from 50 to 100 square feet of direct radiation for each square inch of cross-section in the pipe. If the pipes are short, straight, and high, 1 to 100 would be allowed; if long, crooked, or low, 1 to 50 or more, according to the conditions.
To determine the amount of radiating surface necessary to easily warm a building in all kinds of weather, and to proportion it so that each room will have the required temperature at the same time, are very important points, requiring careful consideration. No rules can be given applicable to all cases, but for ordinary buildings having the average wall and glass exposures, the following table is found in practice to give good results, when used with judgment.