If you happen to invest in a product and it does not fit your head properly, you do have an alternate option. If it is already too small for you, you may be out of luck as it is nearly impossible to enlarge a fabric without ruining it. However, there is a remedy you can take for a garment that fits too large, shrink it. For cotton, it is pretty simple. All you need to do is put it through your washing machine and dryer (on high heat) and it will naturally shrink. But, other materials such as wool need a little more work.
Start off by boiling a large pot of water with a few drops of liquid soap. When it is boiling, reduce the heat to a simmer and submerge the garment inside. From there, take some sort of spoon and stir around the fabric. Once you have done this for a little while, take the beanie and place it in a sink full of cold water. With an additional does of liquid soap, begin to scrub the fibers. This is going to shock the fabric and it will begin to felt together.