Don't worry about growing out of it. If you take reasonable care of it you will have no problem selling it for close to what you got it for. If you do drop it, you will not have spent a ton of money on it so it will not be that bad.
Topflite's response is one of the most mature answers I have seen. I used to joke that I just started riding and thought the ZX14R or the M109R was a good starter bike until I learned how many people truly did just that.
Getting a smaller bike to start with may sound frustrating but the benefits will be huge. As you outgrow the 250 you will be so much better prepared to take on a bigger bike. You will appreciate and have a lot more fun with a 600 after some expirience on the 250. It is not only a matter of getting used to the bike. You also have to take into consideration learning to read traffic, what to look out for and how to handle those situations. Dealing with that on a nimble lightweight bike is going to be far easier than anything larger.
Oh, and that excitement you will get when you first start riding? It never goes away no matter what you are on.