What size mirror goes with a single sink bath vanity?

What about fitting a mirror to cover the available wall space? You mention that it's for a basement so guess that light is at a premium - this way you would reflect whatever light you have got, back into your bathroom & make it appear a lot lighter and larger. You'd have to get the mirrored glass cut and fitted specially, but think it could be worth it. We did this on our bathroom wall and it looks fabulous.

If the vanity is between two walls a full size mirror looks good.

the only difficulty is recepticles have to be cut out if there in the way.. if the vanity has two sinks then you could possibly try two mirrors centered on both sinks...or if the vanity has one sink a mirror centered on the sink looks good also. The size's should be what looks comfortable with you in the opening

I think a mirror that is 24' wide and 30" high should work for you. You don't want it too low (close to the sink), because it gets splattered with water every time someone washes their hands.

maybe a 32'' mirror