I don't know if you have a David's Bridal in your area or if you are interested in going through them, but I know they have a lot of dresses that are both regular and plus-sized. As far as asking her about her size. Just explain to her that you want to make sure all your BMs have matching dresses and you want to make sure she is comfortable in her dress so you need to know what size she needs so you can look into places. As a plus-sized woman, I know there is nothing more uncomfortable than trying to fit into a too-small dress. That way too, you don't get your heart set on a dress that she can't wear, becuase then she might feel embarassed that it's not big enough for her and she'll feel like she let you down. (At least I know I would if I were in that situation). And as far as her wearing a different dress than everyone else, she might feel like the odd one out becuase she is wearing something different. Hope that helps.