For a medicine cabinet, it all depends on what you want to spend. The issue with getting a wider one is that you either will have a very large door to open, that requires you to step back from the sink, or you will have two or more doors to open, with breaks in the glass surface for the door edges, giving you a line in the reflection.
Larger hinged door cabinets sometimes deal with this by doing one larger door in the center and a smaller mirrored door to the side (or both sides). Or there is a crack between two equal mirrors, right down the center. You have to consider how you use the med cab to decide which is best for you. The Quality Bath site has a large selection of med cabs to look at, but their site is closed for the Sabbath from Friday sundown to Saturday sundown (eastern time US). Houzz also has a large collection.
It seems to me like you could have a med cab as wide as your vanity, at most. Think about where you want your lighting, though. Most say that having sconces at face height is best for shaving and make-up, so you would want a narrower mirrored cabinet if you had a sconce on each side. Getting a cabinet with LED lighting right in the glass is another option, and it does not require any extra wall space. This one is even 6 inches deep, so it will hold just about anything you would want to store in it.
Q