I am trying the resize the Canvas by dragging out the blue dots on the corner, but Unity doesn’t let me do it.
How do I increase the size of the Canvas
the UI canvas cannot be scaled in the same way as you would scale ‘normal’ sprites objects etc.
the new UI is bound to the RectTransform and when used in screenspace overlay mode this is directly proportional to the camera screenspace. so it covers the entire screen. in 2D mode it looks massive, but this is to get a better fidelity on the UI as each pixel on the UI is translated into 1 unity unit.
you can see this if you change your screen resolution in the game window, or when changing aspect ratios.
to get a Start button on the UI, you should be able to add a UI > Button to the canvas and drag that around and resize it inside the canvas