Apparently there is some hierarchy as to who gets to wear long and who can wear short, depending on how close you are to the kallah. This corresponds to the order in which she will dance with you.
Also, you want to remember that many shidduchim are made at weddings, so you do want to look good – not over the top, but people will be looking for their nephews and neighbors’ sons…
Finally, have no fear because I hear that there are gmachs for “friends of kallahs dresses”!