I would like to learn to play saxophone. Can I play it from sitting down (wheelchair)? Which is the best beginner size of saxophone? I have small hands and can reach an octave on piano.
The size of your hands shouldn't matter at all. The keys are all right next to each other and don't require a stretch, except for maybe the left pinky--the good brands should have those keys designed ergonomically but sometimes the cheaper brands don't put as much care into them. So check out brands in person and make sure they work.
Alto and lower saxes are typically played resting against the outside of the right leg, though alto and tenor can be played between the legs. I imagine this position would hit the seat of a wheelchair, but knowing nothing further I'll leave it for you to investigate.
If you're having issues due to the wheelchair, then soprano might be the right choice. It's shaped and held like a clarinet. Beginners usually start on alto because the instruments are the cheapest and most people find it easiest to play, but there's no reason you can't start on any size.
Because of the wheelchair, soprano or alto would be your best bet. The alto is typically played with the instrument sitting between your legs, whereas tenor and Bari are positioned to the right.
However... there ARE straight tenor saxophones that help solve this issue! They are a bit of a specialty item, but you should be able to find some. The tenor saxophone is not out of reach for anyone!