Long hair tends to just hang there and look thin, and widen the face. To avoid making your face look larger, try a short hair cut. It will add volume to the hair without making your face look big. It does not necessarily need to be super short, rather a length that improves the shape of your face.
Another thing that helps slim your face is a fringe. This is a great way to try to slim up the face as little bit of fringe suits almost every face shape. However, to look good, it has to be done right. A thick blunt fringe is not a good way to achieve this affect. This will widen the face and make it look larger. A lighter, wispy fringe that creates soft lines around the face works better. Also a side swept fringe is a great way to slim the face. These add dimension to your haircut.
Another interesting aspect to consider is changing your hair partying. There are many different ways a part can be done. A part can be done in the middle of the head, to one side or the other or even a zig zag part or a diagonal part. Try out all the different options and see which one works the best for your face. However, keep in mind that the side parting of the hair the most slimming as it does not enhance roundness.
Ask your hairdresser to give you a layered haircut which is a great way to 'slim' your face. It adds dimension and will also soften the lines of the face. Layers are also excellent for adding volume and texture to the look. A straight, blunt haircut widens the face just like blunt fringes do. Most stylists recommend coloring the hair to add even more depth, dimension and volume. This brings about contrast which is good as it creates shading, which also slims and brightens your face.