This is the measurement across the center of the drain from one side to another.
Standard sink drain size.
This size is precisely the diameter of the pipe instead of the length.
Even if you get a new faucet for the bathroom sink you need to buy a drain of these sizes.
So even though there are different sizes of drains for your bathroom sink according to the international code council the standard is 1 inch.
However most of the bathroom sinks use 1 inch drain pipe.
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.