Hi Jennifer -
While Disney princess dresses are available nearly everywhere at Walt Disney World, the delivery situation you're looking for would be nearly impossible to arrange. Even
DisneyFlorist deliveries are not put into guest rooms prior to their arrival, but rather are delivered after check-in.
I do, however, have some suggestions. You could purchase a dress yourself in advance via DisneyStore.com or another retailer and bring it with you to your hotel. This way you could give your daughter a wrapped box when you get to the hotel. If that seems like too much to pack, you could also give her a Disney Gift Card when you arrive with the intent to go on a princess dress shopping spree when you get to the parks. Disney Gift Cards are available at Disney Stores, most gift shops in the WDW parks and resorts, and at many grocery and discount stores throughout the US. An advantage of getting the dress on site is that she could try it on and make sure it fits before you purchase.
For more information on the princess dress situation at WDW, you may want to head over to the independent website blog.touringplans.com. Search there for an article called "Everything You Ever Wanted to Know About Princess Dresses at Disney World." There is a sizing list there as well as photos.
Good luck with you planning. Please let us know what else we can do to help.