Entire books have been written about embouchures, and it is true that the embouchure is extremely important: small differences in embouchure totally change the feeling and sound of an instrument. However, as with flutes, there is not such a thing as the general ideal embouchure. As with the flute body, it is very much a question of personal taste and preference. This does not mean there are no good and bad embouchures, but that preferences in size and shape are not universal for different players.
Some things can be said however about the embouchure. One thing is the size. Smaller embouchures will produce less volume, but also need less air. The sound will generally be a bit more closed, more reedy, and the bottom notes as well as the second octave will tend to come out more easily. A larger embouchure will need more air, or require a better control of the air stream by the lip muscles. The sound will be more open and brilliant, although also the sound of a smaller embouchure can be approached by covering the embouchure more and controlling the air stream.
Another feature of traditional embouchures is that they are undercut. This means that they are made in such a way that they become larger towards the bore of the instrument. Baroque embouchures are very much undercut, whilst modern flute embouchures often are not undercut at all. On ‘Irish flutes’ the amount of undercut will influence the playing in a complex way. Simplifying things one can say that –within certain limits- more undercut will give more dynamic to the flute; make the second octave sound more easily and in general give a more open sound. However, too much undercut can create problems to make the low notes sound easily, and produce an uninteresting, not reedy enough sound.
For inexperienced players my advice would be to go for a standard medium sized embouchure, or to ask for a smaller embouchure if volume is not that important to you and you want to spare some air.
Unless specified otherwise my flutes come with a standard undercut medium sized oval embouchure. More experienced players can come to the workshop to try different types of embouchures.