There is no rule for the length of dress depending on the time of year...that is entirely up to you! I think it's almost easiest to have some of your girls try both long and short on, that will give you a better idea of which you like. I always
thought long until I saw them on my girls, then I loved short way better!
I would find some that YOU like and send to your MOH to get her opinion, that will help give her a better idea of what you are going for. If you can't pick just one, find a few that work well together and let each girl pick the style they are
most comfortable with from the options you have approved.
It's your day, feel free to make these kind of decisions in a polite way!