I'm 4'11" and have the XS Giant OCR3w (08 version of the Avail). Very nice bike, especially for the money. I've been very happy with it. I'd recommend getting the 1 or 2 version with true road gearing (mine is a triple with an 8 speed cassette and is set up closer to a mtn bike) otherwise I'd buy it again.

I've also test rode the Scott (carbon men's version, very little difference) and was very impressed. It came in second place in my bike search (I ended up getting a Wilier Izoard).

I have absolutely no problems with 700c wheels so long as the bike manufacturer was smart about toe overlap (and I've only test ridden one that wasn't). I have no intentions of dropping down, but if you are used to smaller wheels you may be happier with 650b's.

It seems most mainstream manufacturers are only offering 650b's on the XXS bikes (Giant for instance) that may be a hair too small for you depending on your build (I could ride one, but I'm somewhat long torso'd and like the more stretched feeling the XS generally gives me (plus I like my 700c's)).

Orbea I believe starts 650b bikes a size larger than most do now, so you might want to look into them. (Incidentally this is the brand I had toe overlap issues on, but I was also riding the 700c bike that was technically a size too big for me; I suspect it is not an issue on the 650b bike.)

Ride everything you can get your hands on and have fun. Test riding is one of my favorite parts