I have some relatives like this. Honestly, you were firm, but not firm enough for her personality. Ideally, you would not be put in awkward situations like this, if she was polite enough not to bulldoze and had common sense about younger people wanting her old outgrown clothes, but she is who she is.
If you can just quietly take the jeans and then donate them, fine. IF she tries to make you try them on, say "NO, I don't want to try them on." She cannot bodily wrestle you into the jeans. If she starts a bickering match by pressing you, just keep saying "no" calmly and nicely. She will look like the weirdo. Do not slink off to the bathroom to try them on. Do NOT. It will be a blessing if you make her mad and she stops giving you stuff.
If they're not wrapped under the tree, and she just hauls them out and asks if you want them, say, "No thanks, they're nice, but they're not my style. I wont' wear them." Leave without them. If they're a present under the tree, say thanks and donate them when you get home.
My MIL gives me weird old stuff too. AND she acts like I should be thrilled with an old, torn, smelly, shawl that her high school boyfriend's mom gave her 45 years ago. Like somehow I would find that sentimental. When I said, thanks, this looks ..vintage..she went on and on about it and how much it meant to her....yet...she wasn't keeping it.... I cant' remember if it went in the recycle or trash because it was in such bad shape. But don't get manipulated into trying stuff on. I hear you, It's awful. My MIL kept giving me items to decorate kids room with old curtains from a garage sale, etc. I told her I had taken great pains to decorate the room just how I wanted it and didn't need anything, but she just kept giving us old stuff for the room. It all got donated and she saw the room and saw none of the stuff was there and didn't say anything. We're not close though. I would be gracious about it if she wasn't a conniving alcoholic who is constantly trying to turn my husband against me. You have to look at the big picture. Is she a nice person who you don't want to insult? Or do you really not mind if she doesn't like you?
If it's appropriate, you might even say, "Hey, the hubs tells me you're thinking of giving me your old jeans, Please don't, I'm trying to get rid of clothes, they'll just go straight to the thrift store." It's bold, but these types of people speak no other language.