Asking a 5 year old to pick which princess dress two months before the day she is going to wear it, is going to be a bit like herding cats in my opinion. Of course she is going to change her mind several more times before December.
Are you talking about which dress to bring from home, or are you talking about advising the BBB which dress she is going to pick like they usually ask when you make your appointment. If you are talking about which dress to bring from home, I would just wait until you are packing for your trip. If you are talking about telling the BBB which one, just wait until she gets there and let her pick. The BBB will not hold you to whatever you said over the phone, if you told them one. Your DD can switch when she gets there. My DD has done BBB several times. She has switched from what she told them over the phone, and after the first couple of times when they asked on the phone, I would just tell them I did not know what she would pick. It is not an issue. The idea is that they will try to be sure to have the one in her size if you tell them in advance, but we have been there when they have told parents who had picked one in advance that one was not in stock. We have also seen a child pick on at the BBB that it did not have in her size and they just called and pulled the right size from the Emporium and had the dress sent over.
Are you talking about having to buy one instead of just picking one out of the closet? If so, and you want to go the cheaper route, I would go ahead and have her pick one and order it now. Just tell her they may run out of her size in them and you need to do it now. Chances are once it arrives she will be fine with it- unless she deems it scratchy and wants to send it back. I would go ahead and do it before it gets closer to Halloween. If you wait, they might run out of the right sizes and you may run into restocking issues in November, and then be forced to get one at WDW instead.
The Disney parks dresses run around $65. The cost the same at BBB as in any of the stores at WDW. If you are going to buy one at WDW, you might as well do it at BBB. It is a fun experience. The Fairy Godmother in Training will get it all set up in the fitting room, and then do a little reveal. You don't have to call and change your package in advance. They will be happy to upsell you once you get there!