Enter the measurement of the thing you have. The part comes out to fit it.
Every part that fits onto something real, such as a pipe, a board, a cable, or a bolt, is modeled with the right amount of clearance for that kind of part. Enter the size you measured and it prints to fit on most printers with nothing added.
Printers vary. If yours prints tight or loose, generate a custom size a little larger or smaller than your measurement. A tenth or two of a millimeter is usually all it takes.