Excel lets you resize pie charts, but limits their minimum size. Why, I don’t really know, unless they could otherwise disappear to a single dot that would be difficult to bring back.
Anyway, here is a ‘slight hack’ that lets you shrink a pie chart that little bit further (for example, if you have an interesting background image that you don’t want to have obscured):
- Add a corner legend. This automatically shrinks the pie.
- Delete the legend. This leaves the pie chart at the size to which it was automatically shrunk.