Hehe having been faced with this decision myself recently I know how hard it is to know what to do.
I went from waist length hair to just about collar bone length which was a bit drastic but it was totally the right decision for me. My hair is very fine too so was just flat all the time.
Looking at your face shape, a long bob would suit you - no drastic layers but enough to give it some body. And no, having shorter hair doesn't make your face look fat - You are blessed with cheekbones
Have a chat with your hairdresser before hand - maybe look at some style books and look at some pictures of people with a similar shaped face - that would give you an idea what might suit you. Failing that and you are uncomfortable with going the whole way, then get it cut to half of what you think you want but are worried about.
Good luck anyway
I know when I had mine done it was very liberating and it felt like my head was floating