We need to see your bathroom it is all about context rooms are not designed one thing at a time but a cohesive plan and to see someting in a space nothing like yours does not help. A big question is what is your style . Post pics of your space here in a comment DO NOT start another post then we can help./

The "rules" are really just suggestions and a place to start. Context matters.

Most people choose mirrors that are a bit undersized.

What Lisaaam suggested…always works for a smaller bathroom to feel taller/airier, etc. just have to work out the best approach for sconces or vanity light so the junction boxes are in the right spot for the light fixture.

I don't think you can go to big with a mirror in a small bath.

How small is your space? and what is in it... do you have a window?

We installed a tall mirror in the Powder Room to provide illusion of space. Ceilings 8’ and room roughly 4x6.

A good trick for making a small room look larger is to paint the wall and ceiling the same color. This blurs the perception of where the walls end and the ceiling begins and makes the room feel taller. I would do that and the just get a regular sized mirror. A really tall mirror is just going to highlight how low the ceiling is.

Is the mirror the first thing you see when opening the door? If it is then the bigger the mirror the more behind/ around you you’ll see . It’s really more about making the size of the room the 2nd or 3rd, etc. thing you notice. Distract your brain with gorgeous wallpaper, beautiful tiles, bright window, rich paint colors, etc. before it has the chance to notice how close walls are.