Don't know if there is a "standard" size. We had all our bathroom mirrors custom framed as we needed sizes that fit our own individual needs.

In the powder room that meant a mirror that would work with a 7' ceiling and for dh who's 6'3". Not easy! The mirror is 26" x 38". The vanity is 52" with sconces.

In the kids bathroom we needed mirrors that would also work for much shorter children AND tall parents or guests. Here the vanity is 84" with 2 sinks and lighting above the mirrors. The mirrors are 24" x 36".

The ensuite bathroom has a vanity that is slightly shorter but also with two sinks and lighting above the mirrors which are the same size as in the kids bathroom.

All mirrors have frames that are 2 1/2" or 3" wide. I like the look of mirrors with wider frames but that wouldn't have worked with the function we needed, i.e. work for short and tall people.

I've been doing all those selections this week. What I've done is draw each wall out to scale and draw the vanity and lighting in. Then I make marks for any special functional considerations and see what those needs will require. (Like beatrix was discussing, kids, tall people, people sitting down, etc.)

I draw that in, look at sources of available mirrors to see what's available (unless you're gonna have mirrors custom made) and then draw some different size mirrors in on different photocopies of the drawing. That lets you judge the scale. Your own eye will detect if anything is out of scale. Plus you can measure things on a scaled drawing to check distances, proportions of things to each other, etc.

There may be a quick and dirty way around this, but I don't trust those kinds of "rules" to suit me, esp. functionally. The quick way may be to go to a supplier and ask them your question.