Fitts' Law
Fitts' law states that the time required for a user to move a pointer to a target area is a function of the distance to the target divided by the size of the target. The longer the distance and the smaller the target's size, the longer it takes. In 1954, psychologist Paul Fitts, examining the human motor system. He showed that the time required to move to a target depends on the distance to it, yet it relates inversely to its size. By his law, fast movements and small targets result in greater error rates, due to the speed accuracy trade-off. Although multiple variants of Fitts' law exist, all encompass this idea.
Fitts' law is applied in user experience and user interface design. For example, this law influenced the convention of making interactive buttons large, especially on touch devices, smaller buttons are more difficult and time-consuming to click. Likewise, the distance between a user's attention area and the task-related button should be kept as short as possible.
Command buttons and any other interactive element in the graphical user interface must be distinguished from other non-interactive elements by size. Whilst it may seem obvious, user interface design often ignores that the larger a button is the easier it is to click with a pointing device. As interactive objects decrease in size there is a smaller surface area, requiring a level of precision that increases selection times.
The outer edges and corners of the graphical user interface can be acquired with greater speed than anywhere else in the display, due to the pinning action of the screen. As the user is restricted in their movements the pointing device cannot move any further when they reach the outermost points of the screen; fixing the cursor at a point on the periphery of the display.
Pop-up menus better support immediate selection of interactive elements than dropdown menus as the user does not have to move the cursor from its current position. Therefore, graphical designs that allow the user to interact without moving help to reduce the cursor travel time.
Now you're probably thinking, that Fitts' Law is about bigger is better but actually, it is about usability. There comes a point when increasing the size of an option makes it only a tiny bit more usable. Fitts’ Law shouldn't be used on its own. You should use it in with Gestalt principles, symmetry, and other design laws to create the greatest effect. Knowing how your users will behave removes the guesswork for you, freeing you up to think hard about your design's message. Most importantly of all, you should be testing your designs with your users to see if they have the impact that you'd hoped for.