The length of the handle is really up to you. I watched where I was naturally holding the brush while gluing some paper and marked the handle a bit farther from where my hand sat. The part that I was left with for the handle of the awl was approximately 4 1/4 inches.
After you cut the handle, you can use some sandpaper to smooth out the ends of the paintbrush and of the awl's new handle.
REMEMBER: Be careful when using any cutting tool! Always be aware of how and where are you holding the piece that you are cutting.