Hi @emkjam To develop the sensation of freedom, wider is better but it is different for everyone. Just make sure you can see black between the lips. I must reinforce that this not the size the aperture for all playing, it is about developing a feeling and getting the lips out of the way.
For anyone reading this post, to be very clear, the idea of the aaahoooh is to:
1. Learn to recognise the “aperture corners” and to become familiar with the feeling of the corners firming without clamping down with the centre of the lips.
2. To experience a free flow of air that is to be replicated when playing. This is the same rationale behind recognising the feeling of humming and the Sing C Series i.e. when playing, I want the lips to interact with the air as if they are the vocal cords.