Most medicine cabinets are usually made to fit over one sink and, hence, it's common to see them somewhere around 20 inches wide by 24 inches high. Medicine cabinets that are surface mounted may stick out around 6 inches.
Some cabinets are designed to span a wide counter area. These might be around 50 inches long by 26 inches high. Smaller long cabinets might be 40 inches long by 26 inches high. There are also boxier styles that measure 23.75 inches by 26 inches. On top of those options, you also have oval designs that may measure 25 inches by 32 inches.
To figure out which size works best for your space, start by measuring the stretch of the wall where you want to place the cabinet. That will tell you what kind of room you have to work with. From there, the shape is a matter of personal preference. The right design will jump out at you.