Hello Linda ... there is no rule because ceiling heights differ from home to home. I would suggest to measure the ceiling height 1st, then add the total height of the bath vanity then the back splash and lastly the mirror. Then see how much space you have left between the top of the mirror and the ceiling?

I use this image as a guide not a rule.

You might want to consider sconces on either side of the mirror for more natural looking light. If you use overhead lighting only, the light can cast an unflattering shadow on your face. That also makes it more difficult to apply make up.

Or consider LED lighted mirrors, like this one from Kohler. Saw it in person... amazing! www.us.kohler.com/us/browse/bathroom-bathroom-mirrors-frames/_/N-2elz