Marilyn Monroe (1926 - 1952) was a Hollywood starlet from the 1950s. She was just coming into her own before her untimely death, having completed thirty-one films. She has been an idol for curvy women since she hit her 30s although there is some debate as to the size of those curves. Her weight ranged from 118 - 140 lbs so even at her heaviest she was no bigger than a size 12 (and us girls all know that in the fashion industry anything bigger than a 6 is plus size). Regardless of her size, she was curvy, not scrawny, we still love her.
The most common image we have in our minds eye of Marilyn is probably the scene from Seven Year Itch when she's on the subway grate in her white flowing dress when a gust of air blows up it. This is also the most common costume when women want to 'do' Marilyn. Here's the famous clip to inspire you. It's delicious!
A number of the online shops carry a Marilyn costume which includes the flowing dress in regular sizes. Halloweenie and Fright Catalog both carry it in extra-large. It took forever but I finally found a plus size Marilyn dress under "starlet" at Buy Costumes up to size 3x. It includes the flowing dress, you'd still need a wig (unless you are already blonde) and shoes.
If you're willing and able I suggest forking out the money for a real dress instead of a costume so you get a piece of quality made designer wear to add to your wardrobe that you can wear beyond Halloween or any other costume event. Specifically, I recommend Monif C's convertible 'Marilyn' dress which is actually 10 dresses in one. And it fits sizes 8 - 24. It comes with a high fashion price ($195) but you are also getting quality and wearability. The unfortunate thing is that they don't have white as a colour choice this year.
All you need to do is pick up a wig. The best one both in style and price seems to be at Costume Craze. Add your curves and you are all set to be a Voluptuous Bombshell.
Get your Plus Size Marilyn Costume from Buy Costumes.
Get your Convertible 'Marilyn' Dress from Monif C.
Get your wig from Costume Craze.