Photos of your bathroom will get you relevant advice.

Bathroom doesn’t exist yet. Breaking ground on Monday - but starting to make selections.

Make sure the design doesn't have a wall outlet over the vanity that will limit where you can put a mirror and how wide the mirror can be. That's my first piece of advice.

Other than that I prefer mirrors that are almost as wide as the vanity with lighting the same.

With over the mirror lighting I would probably want 36" for mirror and light over the 42" vanity and 42" over the 48". My feeling is it's a mirror, not a painting, so anything up to full width is not "too wide " in my book.

Not enough information!

How high are the ceilings?

Are you going to have sconces on the sides or a light fixture above? Where are you locating the electrical boxes? Do NOT leave this decision to your electricians!

Most people get mirrors that are too short/squat.

Most of my mirrors are 48 - 52" tall, so I made sure to tell the electricians exactly where I wanted the electrical boxes for sconces and vanity lights.