How much space do you need?
You'll need to consider the size of the bathroom and any fixtures or cabinets already in it. You can get a rough idea by measuring the room and estimating how much space you want to devote to a sink. Keep in mind that you'll likely need extra storage for towels and cleaning supplies.
A bathroom with a small space does not necessarily mean that you have to sacrifice storage or style. If you are looking for ways to save space in your bathroom, then installing a sink instead of a vanity cabinet is an excellent idea. The amount of space needed for a sink depends on the design of the sink -- whether it is wall-mounted or built-in -- and the size of the sink.
If you have more room to work with, such as in a master suite or children's bathroom, it is often possible to install larger and more elegant sinks. This includes undermount styles as well as self rimming and vessel sink options which will be set up over a nice cabinet that can provide more storage space.