M.J.
Recognize that preschool teachers have seen it all, so there's just about nothing your daughter can do that will be shocking or new to them. If she falls and gets hurt, they will be there to give her hugs (and so will the other kids), if she has a potty accident, they'll clean it up. If you've found the right school, the experience should be very positive.
You may be surprised, too, that your daughter finds her voice at preschool. One of our friend's daughters is very small, just like yours, and used to be very quiet. Wow, after starting preschool, now she speaks up, is clear and is far less apprehensive about everything. It's been amazing for her.
The best thing you can do is be very positive with her about preschool, play it up and smile, smile, smile when you drop her off. If she sees your nervousness, it will only make her anxious, so do an Academy Award winning performance. :)