Snapping the diameter (Length or Width) Sizebox handles for cylindrical shapes can be a bit tricky when the shape is placed in center of another cylindrical shape, as shown at 3:35 in this tutorial video. Avoid holding down the [Shift] key until the mouse cursor is close to the correct quadrant point (or cylindrical face) of the other shape. If you, by accident, snap to the center of the other cylindrical shape the dynamic update of the active cylindrical shape will stop and you will have to do it again. This is a quite common problem when you are new to the technique of dragging handles and snapping, but which you will learn how to master quite fast.

Remember that the green colored snap face/edge/vertex function is only active when you hold down the [Shift] key. You don't have to press the key until the mouse cursor is close to the area (face(edge/vertex) you want to snap to.

‍