Her weight fluctuated. In her “prime”, one of the most reliable of her biographers, Anthony Summers, quotes costume designer Billy Travilla's notes that have her measuring 36 C-26-36 at 5′5 and 120 pounds. She worked out quite a bit, too, and had great muscle tone. She also, like pretty much every other female movie star or model of the time, wore a girdle and that cinched her waist in. She looked amazing, obviously, and would probably take a size 4- 6 in today's sizes. Not that different from the average Sports Illustated model.
However, during the filming of The Misfits, she was 140 pounds. Plus size, no. But by no means svelte, as you can see by the photographs I attach. Probably size 10 in today's sizes.
She was, at the end of her life, the slimmest she had ever been (less than 118 pounds, and in the costume tests for “Something's Got to Give”, looking more Audrey Hepburn than MM figure wise imo) and even commented that she “looked better than ever.” It is at this time she wore that flesh-coloured dress that Kim Kardashian famously had to diet to fit into. Sh…