It depends on how wide you open your mouth at present. Get a friend to photograph you saying 'The and.' Then you need to get the picture blown up to life size. Once you've done that work out the radius of the circle your lips forms. Once you have that you can work out the area, pye r squared. An area of 45.45cm3 is ideal. Although you can probably get away with anything from 42.7- 48.3. Next is practice to get to that ideal shape.
Don't throw the picture away, it can be a handy reminder of how you used to say 'The and.'