The sink stopper is the mechanism that blocks the sink drain. The stopper moves up and down when you lift a small rod on the sink faucet that controls a lever in the drain. If your stopper is too large or too small, however, it will not effectively block the drain when you need to fill the sink. If your stopper does fit and just needs replacing, it is easy to find one that is the same size.
Pull up the existing sink stopper. If you simply want to replace a worn-out stopper, take the stopper with you to the home improvement store and purchase one that is the same size, using the old stopper for comparison.
Measure the diameter of the sink drain hole and note it down.
Measure the diameter of the sink drain hole again, this time including the metal flange in which the stopper sits when depressed.
Purchase a stopper with a diameter between that of the drain hole and the metal flange. A stopper of that size will cover the drain hole when you need to fill the sink, and will sit snugly in the metal flange.