28" for sure. I had this dillema a while ago - the only way to resolve it (for me) was to go into H and try on both sizes.
I already have a 32 retourne (souple) Kelly, btw, which I took with me that day. There is a 32 sellier Kelly in the boutique as the "try-on" not for sale bag, and it seemed larger than my 32 retourne simply as it's a rigid shape although it holds the same. My SA seemed to think I should get the 32 sellier but as soon as she brought out a 28 sellier for me to try, I knew it was the right one. I am only slightly taller than you.
There was a craftsman from H in the store that day (on an exchange trip) and he agreed that 28 was right too. It's so feminine and elegant.
I think a 28 is better for a croc Kelly, however you should decide with the two in front of you so you can try for yourself.
One other thing, if you are used to Birkins, you may be more likely to want a larger size as the Kelly closes and the shape tapers towards the top, unlike a Birkin so it may hold less.
You will probably find you can't hold as much as in your Birkin, but its a good opportunity to clear out your purse! I am hoping for a 28 sellier but don't plan on using it as a day bag as I carry a lot of stuff with me.
There's a thread somewhere showing how much each size holds - will try to find it!
Here it is!
http://forum.purseblog.com/hermes/whats-inside-15-20-25-28-32-kelly-253726.html