Measuring your Gas Cylinder

This photo is what you need to do AFTER you perform the next 4 actions.

To expose the cylinder and measure, the 5 leg base with caster wheels needs to be removed. It's very easy.
First turn the chair up-side-down so that the  and  in the center of the base bottom underside can be seen.
Spread newspapers on floor to prevent greasy falling parts from causing stains

Then, 

Then, 

Then, withdraw the base. The base will easily remove from the gas cylinder most likely pulling with it (hidden inside the column) 2 washers and ball bearings. Look down inside the withdrawn column for any remaining part(s) where usually you will find a washer hiding at the bottom. Set the 2 washers and bearing aside to be reinstalled later.

Now measure and select a replacement. Precise measurement is unnecessary.
Instead, you can select a replacement according to your height.

An often asked question-"Whats the difference between a COM and only the cylinder?"
A COM includes the gas cylinder, also includes a bearing kit and new column plastic liner, all preassembled.