Congrats on your engagement!
I feel sure you will get a beautiful ring. It is good that you are doing research.
As the other posters are telling you, it will not useful for us to tell you what size would be 'too small'. I wouldn't like to do that, if only for the reason that if I name 'X' size as too small, there are certainly women reading who have a ring that size or smaller who will then feel bad that their e-ring is not up to some arbitrary standard I (or my possibly shallow friends) made up. I have seen beautiful rings in all sizes. some of the antique rings have center stone diamonds that are .10 or even smaller and the ring is very beautiful with lovely workmanship. Also, what if you decide that you 'must' have a ring with a certain size center stone and then you don't get that size due to finances? Then YOU might feel bad, even though you have a lovely ring which is a gift of love.
You can go and try on rings, looking not just at carat size, but at the design of the ring and the way the different shapes of stones look on you and then you will get some idea of what you will like, on you. Maybe you will decide that size is most important to you and that you would just like the biggest size your fiance can afford (with a color and clarity that assure that it looks pretty and sparkly) and there's nothing wrong with that, of course. You should just get it and enjoy. But we can't advise you about that, because we don't know what your fiance can afford.
You should keep doing research and learn about the four C's and all of that to make sure you don't overpay for what you get.
But maybe you WANT to overpay (in other people's opinion) because maybe you will find that you do prefer the rings from Tiffany's or some other high end place, when you have looked at different rings.
It should be alot of fun looking!!! I hope you have the best time. Congrats again and I look forward to seeing pictures once your start your search (and I'm sure that many others on the board do, too).