Choosing the correct chair.
The most important dimensions for comfort and support are:
The seat width: ( obviously if it is too narrow it will be a tight squeeze and too wide it will not support your arms correctly on the arm rests).
The seat depth: (again if the seat is too deep you will not be able to get your bum to the back of the chair and / or your feet will be raised off the floor. If it is too short in the thigh your feet will not be supported when the footrest rises and it will cut into your calves blocking circulation).
The seat height: (if your feet do not touch the floor it will pull on your thighs and can cause drop foot. Sitting for prolonged periods will also be very uncomfortable. If it is too low your feet will not be supported when the footrest rises and it will cut into your calves blocking circulation).
Other important dimensions:
Back height (if the chair back is too low, there will be no support for your neck and too high may push your head forward depending on the type of back cushion(s) you choose).
Arm height (if the arms are too low it will pull on your shoulders and neck and if they are too high your shoulder will hunch).
How many motors the chair has, ranging from 1 to 4.
What type of back cushion(s) you need button, waterfall, snug fit, pillow or split.