Sizing, in the context of fine arts, refers to the treatment of a surface, such as paper, to prevent it from absorbing too much paint or medium. This is particularly important for watercolor painting, as it allows the paint to flow smoothly and maintain its color and brilliance. Sizing helps to create a barrier between the paper and the paint, preventing the paint from sinking into the surface and losing its vibrancy. Golden Artist Colors discusses the importance of sizing in watercolor paper, noting that too little sizing can cause the paint to feather and lose its brilliance, while too much sizing can cause the surface to resist paint (Source ID: 1).