Household copper plumbing pipes usually come in 1/2-inch and 3/4-inch diameters. That is, if you cut a piece of pipe and placed a tape measure across the pipe's end exactly in the middle, it would read either 1/2-inch or 3/4-inch across. Generally, the main pipes that supply the water are 3/4-inch diameter, and the 1/2-inch copper pipe is connected to it and runs to faucets, toilets, sinks, dishwashers and showers/tubs.
Measure the existing plumbing pipe in your home that may need replacing due to a break or a leak. Take a tape measure and hold it across the pipe sideways (not lengthways).
Hold the tape so that the very end rests against one side of the pipe (the end of the tape has a metal lip to help hold the tape measure in place). Look at the other edge of the pipe and see which marking on the tape lies directly over that edge--it will either be 1/2-inch or 3/4-inch. The measurement you see will be the diameter of pipe you use.
Hold the tape along the length of the pipe that has to be replaced to determine how much you need. If you are installing a new pipe, run the tape measure from the point of the start of the new pipe to the place where an appliance will be installed. Make sure that you account for all possible bends in the pipe, such as in room corners. Always round up your calculations because copper pipe usually comes in 8-foot lengths. For example, if you need 26 feet of pipe, round it up to the next 8 foot increment, which will be 32 feet. This ensures that you will have enough pipe to finish the job if mistakes are made.
Use the same diameter couplings as the diameter of your copper pipe. Just like the pipe, make sure you have extra couplings for all corners and bends, so you can finish the job without making more trips to the store.