The number of a pen sac is equal to the numerator when the outside diameter of the sac is expressed in sixty-fourths of an inch.  A #18 sac is 18/64" across.  This is why most countries have switched to the metric system.

To measure a pen that you don't know the correct size of sac for, there are two things to take into account.  First, the inner diameter of the barrel should be bigger than the outer diameter of the sac; this allows for a little bit of an insulating buffer of air, reducing chances of a leak.  Second, the outer diameter of the nipple be slightly bigger than the inner diameter of the sac, as the sac should stretch to fit rather than simply slide over the nipple.  Usually, the second consideration takes care of the first, but one is still left with a problem-- the measurement for either doesn't have direct bearing on the sac number.  As a rule of thumb, measure the nipple, and apply a sac with a number equal to or less than that measurement.  Since the sac number is measuring its exterior, you're adding the thickness of the material to the measurement of the nipple, and that should provide a good fit.