Because personal computers come in all shapes and sizes, the Start menu, which takes up roughly the same amount of screen real-estate regardless of size, can be extremely useful.
However, on a smaller display, such as that of a compact tablet, this can become annoying. Fortunately, resizing the menu is simple: move the mouse to any corner and drag it in any direction you want. When it's the correct size for your screen, let go of the corner and it'll stay that size. If you want to make more changes or move it back, simply repeat the process until you're satisfied.