Are you planning on carrying on the wedding dress? Where are you planning on putting it? There is minimal closet space on these flights, and usually the only one is the very small first class closet that's suitable for hanging a suit coat, not a wedding gown. If you put it in the overhead expect that others will stuff their bags in on top of or next to it, and it will be bunched up up there as well.
If you do carry it on, it will be your only carry on except your small personal item (a purse or camera bag for example. You will not be allowed a second carryon.
Anne