No, not all tub drains are a standard size. There are a variety of sizes and types of tub drains available on the market, and the specific size and type of drain that is needed will depend on the particular bathtub and plumbing configuration. Some common sizes of tub drains include 1 1/2 inch, 1 5/8 inch, and 2 inch, but there may be other sizes as well. It is important to measure the diameter of the drain opening in the bathtub and consult with a plumber or hardware store professional to ensure that the correct size and type of drain is selected.