The camera lens acts just like a slide or movie projector lens in that it projects an image on film/sensor. The camera lens actually projects a circular image that is quite a bit larger than film/sensor. Only the central portion of this image circle is usable. The outer limits of the image circle are dim (vignette) and have soft focus. Only the central portion of this projected is photographically useful. The central part of the image is called “The Circle of Good definition”.
The diameter of the circle of good definition increases as the lens is racked forward. In other words, the lens to film/sensor distance is at its closest when imaging objects at infinity (∞ as far as the eye can see). As we focus on objects that are closer than infinity, the distance lens to film/sensor lengthens. As we close focus to obtain “unity” (life-size, magnification 1, 1:1 etc.) the lens is racked forward. Now the distances between lens and image is twice the focal length. Additionally, the distance between lens and object is also twice the focal length. The total distance object to image is 4X the focal length. When unity is achieved, the circle of good definition is now twice the size as compared to the infinity position.
OK, the circle of good definition increases as we close focus. Now we must pay attention to the diameter of the extension tubes. If they are too small the corners of projected image will be lopped off (vignette). As a rule of thumb, extension tubes are sized for use with the “normal” lens. A “normal” lens is one with a focal length about equal to the diagonal measure of the format frame. The full frame (FX) measures 24mm height by 36mm length and the diagonal measure is 43.3mm. By convention, the “normal” lens has a focal length rounded up to 50mm. This lash-up delivers a field of view 46⁰ computed in terms of the diagonal. Additionally the circle of good definition just covers the diagonal corners of the frame. It takes special design considerations to make shorter lens that will cover this frame size without vignetting. This is a key ingredient as to why the 50mm lens is considered “normal” for the FX.
Let me add that extension tubes are usually designed around the “normal” lens. If you mount a longer lens, you likely will get some vignetting.