Find a shop and try one on - you want a slightly looser fit than a hat that needs to clamp on in a wind as the cords stop it flying off.
After washing you can stretch the fabric slightly over your knee if needed.
As to slightly small leather hats - soak the hat in warm water and put it on over a Buff to "Wet Mould" it to your head - I have done this with great success with a couple of manufacurers models! It even works with "waterproof" ones....
Dry the leather slowly so as not to invite cracking and give it a good waxing afterwards.
After washing you can stretch the fabric slightly over your knee if needed.
As to slightly small leather hats - soak the hat in warm water and put it on over a Buff to "Wet Mould" it to your head - I have done this with great success with a couple of manufacurers models! It even works with "waterproof" ones....
Dry the leather slowly so as not to invite cracking and give it a good waxing afterwards.