Switch specifications

I'm curious about switches. Most of the time it seems as though the "right type of switch" is implied or passed down through tradition. Most of the Google search results are concerned with switching children, which is not what I'm interested in (frankly I'm not sure whether this will ever go anywhere even with adults), so question for the group:

What are the specifications for a good switch? I know they'd change based on how severe the spanking will be, so share a couple. What I have seen so far has been evidently about any branch on one end to a branch 3 feet/ 1M long and as big around as your little finger on the big end.