like everyone has said its your day, so wear what makes you happy
all I will add is something a friend who recently had a beach wedding mentioned after the event.
she had a big dress, with a train etc and found that she was really hot in the dress, married in the arvo on a fairly warm day and she said it really was uncomfortable with such a big dress, also the sand created chaos with the bottom of the dress and she ended up having to hitch the dress up and carry it, hoop and all.
she found the boning in the bodice was restricting which didnt help matters with the heat.
she said she wished she had chosen a lighter dress to wear