I need to pick out a bridesmaid dress /what size

My daughter is getting married in June and she and with the help of her sisters picked a dress that all the bridesmaids (6 of them) will be wearing from a bridal shop, and they are going for fittings and or measurements (if the dress is not in stock in their size) since they have to be ordered and the bridal shop wants them ordered about 2- 3 months before the date to ensure they arrive and have time for their seamstress to make any needed alterations.

I am assuming from your question that the bride apparently told you to pick a dress you like? If so, if I were you I would try to at least get her input on what color/style she would prefer - if she wants long/short or if she has very dressy or somewhat dressy or beachy sundress, etc. in mind, and go from there. And I would wait until much closer to the date if you are not planning to order it or buy it since you are currently losing weight.

I assume you are asking what size to order it in because you will likely be smaller then than you are now -

My suggestion is wait until the latest you can to order it, then order it in the size you are then. Ask the store what the latest order date is, some require a few months. My wedding gown has to be ordered 6 months ahead of time and this is what I am doing. I am confident that I will be smaller, but I also know life happens.
Most dresses can be altered at least two sizes without affecting the dress. Talk to the people at the store about any issues there might be with taking it down a few sizes.
It is difficult to predict what size you will be even if you successfully lose the weight. It is much easier to take in a dress than to make it larger.