you need to consider the height of your ceilings and location of your light fixture. Take a piece of cardboard and "mock it up" in the space. See if it suits the style.

When I look at this picture, I assume the ceiling is quite high.

24" frame means interior glass about 20-22
I just taped that space off on our current home frameless mirror - WOW is that short!

Set one foot above new 34"H vanity, okay for me at 5'7 cuts off 6fott boyfriends top of head and someone 5'2" will have their chin at frame bottom.

Less than 2" high vanity mirror glass? UGH what were they thinking?!

******************** NEW QUESTION ****************************
9 foot ceiling, 6foot wide wall, 42 wide vanity -
Thinking vertical mirror will make wall seem wider but would horizontal look better with vainty. Thoughts and suggested measurements appreciated....

May I suggest you take a picture of the space you are decorating? IME, the location of the light fixture, if it is going on the wall infront of the vanity, needs to be taken into consideration.

Well, as someone who is 5'2" that configuration would make me crazy!! I like the horizontal mirror, just not the fact that only people between 5'6" and 6'0 can actually see their whole face in it.

This is for a new build, house is framed and attached is room dimensions with typical 30x36 mirror but could certainly go with another size..

I think mirror top shouldnt be below 6' 6" high based on our family height

Vanity light wiring height is unknown (we're 2hrs away from actual construction site) but likely can be adjusted to mirror.

42 vanity is wider than we intended, just had to have this one so dont want mirror to accentuate vanity length. Not sure if vertical makes vanity look like it takes up less wall space or if verticals smaller width ends up making vanity look wider

I think it looks better with the mirror hung vertical rather than horizontal. If you hang it vertical you can put it a little lower but because it then 36 tall instead of 30. It will be the right height for both tall and short people. You can the put art work or shelves to the left of the mirror.

Thought you might like to see an actual photo of a small mirror hung vertical.
This mirror is 24x36 and the sink base is only 28 inches wide. We debated whether to put a plate mirror across the whole wall but opted for the smaller vertical mirror and the decor items.

Nice bathroom - we have two narrows vanities like this upstairs (28 & 30) which work well like yours with the vertical.

The sale ends today but definitely passing on the matching mirror -that 24x36 is too narrow horizontally to fit people of different heights, also to narrow vertically for a 42" wide vanity.

With time pressure over to buy the matching one while on sale, going to wait til the vanity gets installed first. Leaning towards vertical but with vanity wide for room, want to tape off wall to see it in person. .