The size of the guitar stool doesn’t really matter. You ideally want a stool that allows for adjustments, so that you can decide for yourself if you feel better resting your feet on the ground, or with them slightly up. The main idea here is that the guitar is going to rest in the same position anyway.
However, the strap of the guitar should always be carrying the weight, and the guitar should be as low as it normally is when you stand up and the strap is around your neck. Once you have been playing for a while you will get accustomed to a certain position. The disadvantage of buying a stool online is that you don’t know how it feels.
Try out more music stools for guitar and get a feel of what feels right for your body and what doesn’t. Most guitar stools and chairs are around the same height anyway.