There are many, many paintball harness on the market and still in circulation. Not all belt lengths are the same, nor are there sizes like pants or underwear, not all brands are the same either and size charts can be very hard to find if the exists. This table should help you pick out a harness.
Length: Measured in inches. Example below.
Stretch Factor: How much the belt stretches. This number represents how much the belt stretches under normal operating tension. A harness with a Stretch Factor of 4, means that each side of the belt stretches an additional 4 inches. A belt with a length of 50in and a stretch factor of 4 means that stretched out the total belt length is 58in.
If you measure 40in for the most part a 40in harness will work as the Stretch Factor helps. keep in mind that some harnesses with a 38in may also fit like a 40in because they have a higher Stretch Factor.