If you are wanting a canvas painting, this may help you pick what type. Then once you pick your type of canvas, you then pick what size canvas you had in mind.
Gallery wrap: is a method of stretching an artist's canvas so that the canvas wraps around the sides and is secured to the back of the wooden frame. The frame is usually 1.25" thick. As a result, the hardware (staples or tracks) used to secure the canvas is not visible. The sides of the canvas are prepared and primed in the same manner as the face. They may then be painted a solid color or painted to continue the image appearing on the face. This method of stretching and preparing a canvas allows for a frameless presentation of the finished painting or photograph.
Do not need to frame.
Traditional Canvas: Canvas is typically stretched across a wooden frame called a stretcher, and may be coated with gesso before it is to be used; this is to prevent oil paint from coming into direct contact with the canvas fibres, which will eventually cause the canvas to decay.
Typically you frame, but do not have to.
Canvas Board: A flat panel made of wood
You would normally frame.