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

Back Height =~9"
Back Width = ~13"
Seat Width = ~17"
Seat Depth = ~14.5"

Gas Lift Sizes

HeightSeat Height Total Height
Standard~17" - ~22"~30" - ~35"
Small~15” - ~18”~28" - ~31"
Tall~19.5” - ~27.5”~32.5" - ~40.5"

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.