Hormones can have several effects on the face many of which I can simulate in your Virtual FFS if you want me to:
They make the apple of your cheeks fuller and rounder and this can be quite strongly feminising. Incidentally, it seems quite possible that the lips become slightly fuller on hormones for the same reason but that is purely speculation at the moment.
They can narrow the jaw - they don't affect the jaw bone but they do reduce muscle mass and when this affects the chewing muscles (called the “masseter” muscles) it can narrow the jaw a little at the back. The effect is more likely to be noticeable if you have fairly strong chewing muscles to begin with.
3. Skin Texture
Hormones make the skin texture softer and more feminine. This is a much more powerful effect than many people realise - the best way I can describe it is that it gives the skin a feminine glow.
4. Skin Colour
Females from all ethnic groups have slightly paler skin than males. Some scientists think this helps women produce extra vitamin D from sunlight when they are pregnant. I can find no studies that have looked into whether estrogen makes your skin a little paler but based on my own experience and observations I suspect that it does.
5. Sub-cutaneous Fat
Hormones give you an extra layer of fat under the skin and this can help round off some of the hard edges associated with a masculine face.
6. Beard Reduction
Some people find that their beard becomes less thick on hormones.
7. Muscle Moustache
The muscle moustache is a term I have come up with to describe the heavier musculature men often have in the moustache area and around the mouth. Read more on this in the page on lips. Hormones can help reduce this by reducing the muscle just as they do with the jaw but they also fill the cheeks as I mentioned above and this creates less of a contrast between the muscle moustache and the cheeks.
8. Psychological Effects
It is very important to consider the psychological effects of hormones. They tend to have a calming effect which is not really relevant to FFS but what is relevant is that the physical effects of taking hormones can make you feel differently about yourself. We often start our transitions in desperation, feeling grotesquely masculine and eager to have any feminisation procedure going but if you find that after some time on hormones, you pass well, you feel more physically feminine and you feel a lot happier, you may find that some of the procedures you thought were essential no longer seem so important.
The changes I've listed here are really very powerful when combined, especially when beard removal is included and many people don't realise just how much difference they can make to your facial femininity. So I often suggest that it is probably best to be on a full dose of hormones for a good year or so before your FFS. Of course, this is not always possible and each case is different. For example: some people have facial masculinities that are clearly going to cause problems despite any hormonal effects - strong brow bossing is a typical example. But in more borderline cases, especially with regards to the cheeks and the jaw, it might be worth taking a cautious approach.
I have seen little evidence so far that FFS surgeons are aware of the changes to the chewing muscles from hormones and I regularly see surgeons recommending cheek implants for TS women who have not yet started hormones and whose cheeks are likely to improve a lot when they do take them. Be aware of this - I personally don't feel that FFS surgeons should generally be recommending cheek implants unless they know about your hormonal status and have taken any potential changes into account.