This size is precisely the diameter of the pipe instead of the length.
Standard sink drain size.
The standard size and setting every state may have different code on plumbing and it may affect the required size of the bathroom sink pipe.
This is the measurement across the center of the drain from one side to another.
The drain strainer the bowl shaped piece inside the drain mouth matches this dimension.
Even if you get a new faucet for the bathroom sink you need to buy a drain of these sizes.
However most of the bathroom sinks use 1 inch drain pipe.