1 1/2 or 2 inch. 2 inch is better because it give more venting for the drain.
The proper size minimum size is 2" and ideally there would be an air break which is not always practical with a 2" stand pipe.The "vent" is installed AFTER the trap to prevent air locks and 2" is used to allow the Volume to flow freelyThe size of a standard sink drain pipe is typically 1 1/2 inches in diameter.
The recommended size for a sink drain pipe is typically 1.5 to 2 inches in diameter.
2"
The standard kitchen sink drain pipe size is 1 1/2 inches in diameter.
The standard shower drain pipe size for residential bathrooms is typically 2 inches in diameter.
A standard sink drain pipe is typically 1 1/2 inches in diameter.
The standard shower drain pipe size for most residential bathrooms is 2 inches in diameter.
A 1.5 to 2-inch diameter pipe is typically used for a sink drain.
The standard washer drain pipe size for residential washing machines is typically 2 inches in diameter.
The recommended drain pipe size for a residential kitchen sink installation is typically 1.5 to 2 inches in diameter.
The recommended washing machine drain pipe size for optimal performance and efficiency is 2 inches in diameter.
The standard kitchen drain pipe size for residential plumbing systems is typically 1 1/2 inches in diameter.