Use a tape measure to find the diameter, in inches, of your copper pipe. Hold the pipe so that it is perpendicular to the floor or another flat surface and measure from one side of the circular opening to the other. For a proper measurement, the tape measure needs to cross the center of the pipe's opening.
Find the total length of pipe you will need to use in your heating or cooling application. This will be the length, in feet, of the distance between your heating or cooling unit that the point where the pipe ends and the heat is outputted into your home. This length will vary wildly depending on exactly what you are using the copper pipe for.
Compare the diameter and length information you measured in the previous steps with a British thermal unit size chart for copper piping, like the ones found on Comfort-Calc or Engineering Toolbox. For example, a 1/2-inch diameter pipe has a maximum capacity of 15,000 Btus. Compare this with the Btu output from your heating or cooling unit to determine whether you need a larger or smaller pipe, or if the pipe you already have will work. Likewise, you can determine the ideal diameter of your pipe from the length. For example, if you need 180 feet of pipe you need a 3/4-inch diameter pipe, which has a 40,000 Btu output.