Here's the thing ... you need to remember that the shower is a GIFT to her. And it's not a mandatory gift, either. It's something you're choosing to throw for her. She's not entitled to receive a shower, you're not obligated to throw one for her, and she's being incredibly rude to dictate what kind of gift you should be giving her.
Furthermore, you have every right to say NO to her once in a while. She doesn't get to have everything she wants just because she's the bride. You're not a bad bridesmaid, a bad friend, or a biittchy person if you put your foot down when she gets ridiculous.
So here's how you handle it:
If she starts telling you that she wants a specific restaurant, pricey decorations, etc., just laugh and say, "Bride, relax! We've got something very lovely planned for you, so don't worry about it. You'll love it!" Then change the topic or just end the conversation by hanging up the phone or walking away. If she keeps making more demands, then completely ignore her.
If she hands you a 100-person guest list when you only budgeted for 40 people, then say, "Bride, we can't accommodate 100 people. I need you to cut this list down to absolutely no more than 40 people. Get it to me one week from today so that I can send out invitations." If she complains that she wants more than 40 or that she simply can't cut it down to 40, then you should sweetly reply, "There's no way that we can host more than 40 people. It just isn't going to happen. If you don't want to cut the list yourself, then I can either pick 40 people myself, or we can just cancel the shower all together. It's your decision. Either way, I need to know exactly one week from today, or else we won't have time to get everything together." Again, change the topic or walk away after that.
The key here is to not give her any input. Because, frankly, she shouldn't have any. This is a party being thrown in her honor, and you are 100% correct when you say that her only input should be the date and a guest list within your means. It's incredibly rude for her to tell you what kind of party to throw for her, and it's breaking all kinds of social and wedding etiquette.
Don't let her tell you what's going to happen. YOU need to tell HER what's going to happen. The only choices she should have are to (a) do it YOUR way, or (b) not get a shower at all. Just tell her what's going to happen, and then tell her that she can either accept the lovely shower that you have planned for her, or she can decline the shower entirely and she can get NOTHING. Dollars to doughnuts says that she'll shut up and leave you alone, if the alternative is that she gets nothing. She sounds like a spoiled brat, but you are not obligated to put up with her greedy attitude just because her parents did a crappy job raising her.
If she continues her whining and moaning that the shower isn't good enough for her, or that you need to do it her way, you need to interrupt her, look her in the eyes, and say politely but firmly, "Bride, listen to me. The other bridesmaids and I are your friends, and we are trying to plan something really nice for you. You need to realize that there are limits to our time and our budgets, and you can't possibly get everything that you want. And, frankly, it's hurting our feelings that you keep making demands of us, because it implies that we aren't capable of throwing a nice shower for you. So you need to either trust us to do this right, or you need to decline this shower if you feel like it will never be up to your standards. It's your choice." And then walk away.
If you can't talk to her in person about this, then do it over the phone. DO NOT, DO NOT, DO NOT email her, text her, or do this over Facebook. It MUST be in your own voice. The written word can get misconstrued (and forwarded to lots of people, and maybe even changed without your consent). You need to buck up, be an adult, and TALK to her. If she's truly your friend, then you should be able to talk to each other.
Hopefully she will come to her senses and apologize for her crappy behavior, or at least shut her mouth and enjoy the shower you're planning for her. If she throws a fit at this, or stops speaking to you/kicks you out of her wedding, then she's a terrible person and you're much better off without a "friend" like this.