I started on a 600, and i will not say not to start on a 600, a 250 will kill you just as quick as a 600. But i will say you better respect the bike when you get one, because they will bite your ass as soon as you give them half the chance.
At 5'6 you will have to put lowering links on them to touch the ground, once you become a better rider you can raise them back up to get all your handling back. i have a friend that is about 5'6 and he rides a 636 stock height, but he has been riding for several years, he can bairly tip toe the ground. wait and check out the 06 gixxer 600, i think the seat height is dropping about an inch (just like they did the 1000) so it might be a little easier.
A 250 ninja would fit you good, and would be PLENTY fast for a good year or so of riding, and they hold their resale value good, plus they are cheap, and expect to drop your first bike a few times, (don't wanna drop a $9000 bike, that cost about $1000 to fix every time it hits the ground, or more) and don't feel like you will look stupid if you ride a 250 ninja, alot of guys, and girls in our group ride them, hell i even get on one from time to time.
But what ever you chose, just be carefull out there, cause people are crazy these days, in cars and on bikes.
Hope that helps ya some, good luck with your bike purchase, oh yea take the MSF class, it is well worth it.