According to signature hardware a standard kitchen drain is 3 1 2 inches in diameter.
Standard sink drain size.
This is the measurement across the center of the drain from one side to another.
So even though there are different sizes of drains for your bathroom sink according to the international code council the standard is 1 inch.
This size is precisely the diameter of the pipe instead of the length.
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.