When choosing a pop-up drain for your sink, the main factor to consider is the size of the drain hole. The most common size for a bathroom sink is 1-1/2 inches, but you'll also find drains with smaller or larger openings. A drain with a 1-1/4 inch opening can be used if your sink doesn't have an overflow hole. If your sink has an overflow hole, you'll need a drain with a matching opening (usually 1-1/2 or 2 inches).
Once you know the size of the drain hole, you can choose a pop-up drain that's compatible. Be sure to check that the flange (the part that mounts to the sink) is the same size as well. Most pop-up drains come with an adapter that allows them to be used with different sized flanges.
If you're not sure what size pop-up drain you need, take a look at our comprehensive guide. It includes everything you need to know about choosing and installing a pop-up drain for your bathroom sink.