You can do one that’ll take up majority of the wall above the vanity; so approximately 48”
Or you can go smaller and center it with the vanity.

If you do the larger one, you can do a light fixture/ sconce above the mirror.
And if you do smaller and center it with the vanity, you can do sconces on either side of the mirror.

https://www.westelm.com/products/seamless-triple-medicine-cabinet-b3982/

https://www.westelm.com/products/arch-double-wide-metal-framed-medicine-cabinet-b4297/

https://www.rejuvenation.com/products/grae-inset-mount-medicine-cabinet/

Medicine cabinets aren't dated! They are a practical and timeless storage solution.

You can do a bit larger than sink width or a bit smaller than vanity width. There isn't just one way of doing this.

If you are gutting the bathroom. then do the work to get the medicine cabinet recessed into the wall.

My 24x48 recessed Robern medicine cabinets over each sink.