Alot of the question you are asking depends on your experience of being on diets. If you know you are likely to be good and stick to it, know that you generally lose weight quite easily then yes, order the dress now in the size you aim to be.
If you think that ordering your wedding dress in a certain size will be the catalyst you need in order to MAKE you lose weight then you will be severely dissapointed. Sometimes this can actually cause weight gain due to the psychological aspect of the pressure involved.
The other question is do you NEED to lose weight? Too many brides put pressure on themselves to become something or somebody they are not just for a day that is their wedding. Fine if it will improve your health and wellbeing, but remember your partner proposed to you as you are, not expecting you to change anything.
Hope this helps
