Is there a recommended smallest button size under normal conditions?
By "recommended" I mean prescribed by some document like:
By "normal" conditions I mean:
Microsoft's UX Guide for Windows 7 and Vista recommends:
"Make click targets at least 16x16 pixels so that they can be easily clicked by any input device. For touch, the recommended minimum control size is 23x23 pixels (13x13 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.
Well, I try to make important/common mouse targets as large as possible without looking bad, something about 20 pixels (assuming 96 DPI) height, and as much width as needed to accomodate labels. If the button has no labels, which is very rare, I found out it's actually comfortable to have an aspect like 20w/50h (with the icon on top, not center), since the mouse is easier to move horizontally. So it's also good to keep them in the same row.
In addition to what MsLis suggested the UX Guide also suggests a minimum width of 75 pixels specifically for Command Buttons.