1 Measure from the most posterior point of the body to the inside of the knee, minus at least two inches. Some prefer more leg overhang to make room for their hand when lifting their leg.
2 Measured from the seat base to the top of the chair back. Depends on how much upper back support is needed, and also affects freedom for the upper body to rotate.
Rear Seat to Floor
3 Measurement from the ground to the rear seat edge. Relative to the front seat-to-floor dimension, this determines the rearward slope ("dump" or "squeeze") of the seat.
4 Determines how far the toes extend away from the body, measured from the horizontal. A tighter angle allows the chair to turn around in less space. Depends in part on ability of the knee to bend towards the perpendicular.
5 Determined by the widest point of the body from knee to hip, plus an inch to ensure room to move. Consider bulk of clothing, particularly a heavy winter coat, if relevant.
6 Angle of the wheel relative to the vertical. More camber improves stability and agility, but also limits ability to pass through narrow spaces. A typical daily chair uses three degrees of camber.
Front Seat to Floor
7 Measure the leg from the back of the knee to the sole of the foot. Then subtract the thickness of the cushion when it is compressed. Next, add a minimum of two inches for footrest clearance. Do not add the footrest clearance if the chair will be foot-propelled.