File textures are bitmap images that filter better than most procedural textures and can result in better image quality.
For more information, see Baking illumination and color.
You want your texture to fit your model perfectly, as though the texture has been painted on. You could measure the surface to which the texture is attached. However, because Maya Creative is more efficient if it scales any non-square texture into a square texture, using a square image to begin with is recommended. If the object you are texturing is not square, for example, a lamp pole, place your texture in a black square.