There is no dearth of choice if you are looking for Halloween costumes for plus size women. A great variety is available in online stores, and there are a lot of costumes that you can easily make for yourself.
Superheroes come in all sizes, so there is nothing stopping you from throwing on that cape and pretending to be Superwoman on Halloween. You can even find the other bits of the costume easily: a sparkly red skirt would do well and is easily available in any store, and tops with superman logos can be found at any clothes store.
Dressing up as fairytale characters also does not lose its charm. Putting together a costume such as Red Riding Hood is rather easy, and can sure be fun. Thrift stores will probably be where you can find a lot if useful gowns to work well for such costumes. You can always buy a white or cream dress from thrift stores, and then dye it the right colour to wear on Halloween. Some other ideas that are popular are Dorothy from the Wonderful Wizard of Oz, a vampire from any of the many vampire books there are etc.
Animal costumes are also rather popular every year. This year, you could go with the latest trend and dress up as a Fox. Cat costumes are also available in plenty. In fact, you can make one yourself very easily: simple wear a black skirt and top in any style you like, make some ears out of cloth and sew them on to a hairband. The tail can be a little tricky, but you can always buy some black faux fur and stitch it around wire to give it the right shape.
There is really a lot of inspiration to be found for Halloween costumes around you. For example, think back to your favourite cartoon when you were a kid. You could dress up as Betty or Wilma from the Flintstones, or as Granny from Looney Tunes, or even Morticia from the Addams Family, to go well with the Halloween mood! Just use some imagination and you will be able to come up with the perfect costume to suit you.