Question

Is there a recommended smallest button size under normal conditions?

By “recommended” I mean prescribed by some document like:

• Apple HCI Guidelines
• Windows UX Guidelines
• or some ISO standard..

By “normal” conditions I mean:

• desktop/office use
• standard 96dpi monitor resolution
• mouse/touchpad for pointing (no touchscreen)
• non-disabled or visually impaired users
• standard “theme” (no large fonts/icons)

Answer 1

Microsoft’s UX Guide for Windows 7 and Vista recommends:

“Make click targets at least 16×16 pixels so that they can be easily clicked by any input device. For touch, the recommended minimum control size is 23×23 pixels (13×13 DLUs).”
   where
“A dialog unit (DLU) is a device-independent metric where one horizontal dialog unit equals one-fourth of the average character width for the current font and one vertical dialog unit equals one-eighth of the character height for the current font. Because characters are roughly twice as high as they are wide, a horizontal DLU is roughly the same size as a vertical DLU, but it’s important to realize that DLUs are not a square unit.”

You may also want to look up Fitts’ Law, which calculates the time necessary to complete an action as a function of the target size. That can help mathematically determine the trade-offs of different button sizes.