I bought an Aeron Size A because I'm 5'4" and thought that's what short people get. Set it up, sat down, and my thighs hit the seat edge within an hour. The seat pan is short, yes, but the whole chair scaled down. Returned it, paid $90 in shipping, and reordered a B.
Here's what I wish someone told me before I clicked buy: Size B fits a wider range than you think, and your thigh length matters more than your height.
Size A Is Really for Short Thighs
The seat depth on A is about 16 inches. If you have long femurs relative to your height, your knees will overhang and the front lip will press into your thighs. That's the pain I felt.
Sit on a flat chair, butt against the backrest, and measure from your butt to the back of your knee. If that distance is under 16 inches, A might work. If it's closer to 17–18, like mine, go with B. I know a 5'2" friend who fits A fine because her femurs are short.
The Armrests and Back Height Are Also Compact
A has a lower back — about 21 inches tall — and armrests that sit lower. If you have a long torso, your shoulders won't get support. I felt like I was cradled more than supported.
Raise the chair, lower the desk, and you'll notice the armrests can't get high enough if you're average-torsoed. Test it with a measuring tape on your current chair: the distance from seat to elbow. If it's more than 10 inches, the A's armrest range will top out.
Size B Is the Common Fix
After returning A, I ordered a B. Seat depth is 17–18 inches, back is taller, armrests go higher. It fits my 5'4", 145lb frame comfortably. I know people under 5'2" who still choose B and like it because they prefer more seat surface.
Don't guess. Sit in one if you can, but if you can't, measure your thigh and torso. If you're under 5'2" with short femurs, go A. If your thigh length is 17 inches or more, go B.
And don't buy used without checking the cylinder and mesh — I did that with my first Aeron and ended up replacing the gas lift.
Before you commit to any chair, measure your desk height too. I once bought a 60-inch desk that was too tall for the room, and that's a different kind of mistake I wrote about. For beds, similar logic applies — you can't just trust the label. The King Size Bed Base Mistake I Keep Seeing Buyers Make is about measuring your room instead of assuming.
The One Factor That Decides the Buy
Your thigh length, not your height. Measure it, compare to the seat depth, and pick A or B accordingly. That's the only way to not waste $90 in return shipping.