Since they’re paying for them it would be nice of you to ask them to wear something they feel comfortable in. I know that it’s your big day but you don’t want to humiliate your closest friends and relatives, do you? If this thought is based on the fact that you want to be the center of attention then don’t have any bridesmaids at all.
When shopping for dresses have a clear idea of what color and style you want and make sure as many of your bridesmaids are with you to try them on. See what they look like and if they are comfortable in them -- can they easily walk, do they have the coverage they want or need, can they dance the night away with you and your new hubby?
Remember that the single gals could seek revenge on you when it’s your turn to be their ‘maid or they could all get back at you at the shower or bachelorette.