Please note that the seat height is only defined by the gas lift and is the same for the Petite Gokhale Pain-Free Chair and the Standard Gokhale Pain-Free Chair.

The Petite chair has a smaller seat pan and a smaller and lower back rest.

The standard chair height is designed to cover the needs of a broad range of heights and sizes.
A good way to determine your correct size chair is to find a seat that is at the right height for you, and to measure from the top of the seat to the floor (try building up seat height incrementally with a stack of books or magazines). Then you can pick the gas lift size that includes that height, with some room in either direction.

It is important to remember that having your arms at the right height for your work surface is an important step in maintaining good posture at a desk or table. When your hands are resting on your work surface—be it a keyboard, table, or other—and your shoulders are rolled back and relaxed, your arms should form a right angle at the elbow: upper arms hanging straight down and forearms parallel to the ground, wrists in a neutral position. Your chair should be pulled in as close to your work surface as possible, and your keyboard, plate or other object placed close to the edge as well, to eliminate the need for reaching or leaning forward.
If the right height for your arms does not allow your feet to comfortably rest on the floor, you may need to make adjustments to your work surface, such as raising your chair and elevating your keyboard, or lowering both your chair and your work surface (perhaps by getting a new one), or creating a small footrest for yourself. The pronged feet of the Gokhale Method Chair base can also be used to rest your feet on, and allows for a comfortable transition from stacksitting to stretchsitting, which shortens the available length of your legs. You can additionally add a footring to the chair which allows your feet to rest ~8 inch above the floor.