Is it improper for me to tell one of my bridesmaids that she can't bring a guest?

Just tell her "im sorry but we really don't have anymore room to add extra people, i hope you can still be a part of my wedding and i would really love if u just come and enjoy it". That's all. Sometimes we get so caught up with trying to please our guests that we forget to do what is necessary and what pleases us. Im not saying that you have to turn into a spoiled bridezilla with tantrums yelling "it's MY wedding!!!!!!" but im saying if you can't accomadate a extra guest then don't. For my wedding I couldn't allow people to bring extra guest unless they were married or in a very serious relationship(which is the only time its proper to allow them to bring a guest). But other than that,everyone whose coming already knows some of the other guests who will be at the wedding. So she should understand