The dimensions of the Petite Gokhale™ Pain-Free™ Chair are:

Back Height = 8"
Back Width = 13"
Seat Width = 17"
Seat Depth = 14.5"

Gas Lift Sizes

HeightSeat Height Total Height
Standard~17" - ~20.5"~33" - ~36.5"
Small~14.5” - ~17.5”~30.5" - ~33.5"
Tall~19.5” - ~27.5”~35.5" - ~43.5"

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.