You need to first have a heart to heart talk with your fiancé about this. Tell him how much you love your future mother in law and how much you appreciate her involvement in the wedding preparations, but also explain to him how important it is to make your own choices as a couple, deciding on how you want your wedding to look, feel and be. You will need his help to get your message across to her.
If there’s anybody who knows how to best talk to her, it’s your fiancé. Eventually, you will have to sit down with her (but not in an intervention kind of way) and have a relaxed, casual discussion on the wedding details. Hear her out. She may have wonderful ideas that might interest you. Don’t make her feel like all her suggestions are not welcome. If there’s something you disagree on, let your fiancé take the lead in clearly explaining your side. I’m sure your future MIL has the best intentions, and her wanting to make everything grand is because she truly loves the both of you and is overwhelmed with pride and joy. I feel for you. It’s your wedding. But you also have to learn to compromise, especially in this situation.
Remember, marriage also means the union of two families, so you’ll definitely be seeing more of her. You don’t want your married life to start on the wrong foot.