Universal Principles of Design: Fitt's law

This is my favourite principle: "The time required to move to a target is a function of the target size and distance to the target".
In web mapping, it is often needed to select feature (to open a popup or present attributive information, for example). If the representation size of the feature is big, then it will be easier and faster to select it.
Another example is the size of the icons: a 16x16 icons will be more difficult to click than a 32x32.
Of course, it's not possible to present only huge icons and huge features, but for common and repetitive operations, they should be used.
Source: "Universal Principles of Design" from Lidwell, Holden and Butler