Seriously?! Size 12-14 is not that big! You can probably find any costume you like in your size.

Go as Sookie Stackhouse from True Blood. Wear black shorts and a tight white shirt (write Merlotte's on it), a black apron, and little tennis shoes. Put your hair up in a ponytail.

I tend to go about costume hunting in the opposite direction than most. I find a piece of clothing that I love, would be comfortable wearing, and might not purchase/wear otherwise- and go from there. I'd start by finding a piece of fun/interesting clothing that fits you nicely and makes you feel awesome and then make a costume out of it.

Something obscure might be more fun and less likely to get snarky comments (even petite girls get them). A plant, mineral, creature, theory, pop culture reference, book character, one of your more notable friends, a grandpa (I love sexy weird crossdressing on Halloween), one of your pets, a household object, something totally made up (I was a bearmouse two years in a row). Start off by finding something that makes you feel sexy and build from there.

Is Viking as cliched as pirate wench?

Here's where I'd go--
Torrid Online:
http://www.torrid.com/torrid/Collections/LookBook/Halloween.jsp

And Hips and Curves Online
http://www.hipsandcurves.com/plus-size-lingerie/plus-size-costumes-fantasy-c-51.aspx

Both these places have costumes ranging from cute to out-and-out SEXY, in sizes 12--on up to extended sizes.

Here's another idea-- over-sized tailcoat, white shirt, baggy pants and tie (or bow tie)-- with the addition of a Bowler hat-- with fake dreads and makeup, you're Boy George, with short hair and appropriate mustache, Charlie Chaplin. Spats would really make the costume, and maybe even add a walking stick. Sometimes, gender-bending can be sexy!

Have fun!

I just saw a really cute size 12 disco mini dress online. Pair it with an afro wig (just bought one for 6 bucks!) and some go-go boots and you're set! How hot is that?!

'Sexy' is in the eye of the beholder: Borg Queen, Mother Nature, Catholic School Girl/Teacher, Cowgirl, GI Jane, Santa Elf, Elvira/Vampirella, Snow White, Southern Belle, and the classic Black Cat are all various degrees of sexy to various people. Only the school girl & elvira would likely need to show a ton of skin/scantyness.

some thoughts:
Sexy Transformer: not scantily clad at all, and the car choice can make it sexy / funny: go as, for an example, as a 'Ferrari with her top down'
con: Requires significant DIY skillz.

Basketball player. Come'on: the Seattle Storm are hot right now! Go as Swin or Svetlana or Sue!

Boy Scout - there's something about a girl in a uniform, right?

Go as the Goddess Artemis - flowing robes cinched in at the waist to take advantage of your curves, and a sword and bow to discourage rude comments.

Cleopatra? http://www.displaycostume.com/store/product.php?productid=58881&cat=635&page=2

Depending on how your weight is distributed, and what you plan to do for the evening, there are many costumes to choose from. You could put together something Victorian and go as Jill The Ripper, you could cover yourself with glitter and go as a Twilight Vampire, you could dress as a nurse, or get a prom dress and soak it with blood like Carrie. The key is to find something that you feel sexy wearing - and sexy is more about attitude than wardrobe.