The most popular costumes for plus size women fall into the sexy category. This usually means incredibly short and baring a lot of skin. This may not be appropriate for greeting the kids at the door, the office party, or dancing the night away with friends.
You can go with traditional costumes like vampires and witches (my personal favorites) or pirates and goddesses which are always available in many styles. The good thing about costumes like vampires and witches is that they can usually be recycled into a new costumes the following year because they are so similar in function. One year you can be a witch in full garb and the the next you can lose the hat and broom, change up the make-up and be a vampire, or gothic princess.
If you have the disposable income there are always the trendy costumes that change every year, usually based on currently popular TV shows and movies -- the most popular are super heroes and their sidekicks -- the downside of trendy costumes is that they do not usually come in a large range of sizes and are listed as "plus" but are really only about a 1x.
When shopping for costumes online read the sizing information carefully. It is OK to have a costume turn out to be a little big because you can always adjust it yourself with pins or sewing but a small costume will be worthless to you and you do not want to be caught without because of return shipping times.
If you are a size 18 or under chances are the Queen/Plus size will work for you but if you are larger look for costumes that come in multiple size ranges like 1x/2x or 3x/4x.
When it comes to Halloween costumes, remember they do not have the same durability as regular clothing. Plus size people have to keep their stress points in mind when choosing a costume. If you have wider hips then it is probably not a good idea to choose a costume with a straight cut dress as the stress points where your hips meet the seams of the fabric may not hold old with the pressure of bending or sitting. Instead go for costumes styles that have an a-line that will comfortably accommodate the hips. If your arms have a larger proportion then do not choose a costume that looks like it has form fitting sleeves. Either go for a sleeveless style or and open shoulder style to reduce any possible stress points here.
If you keep these thoughts in the back of your mind you will have a successful journey through the world of plus size costume shopping.
Happy Halloween!
