Our vanity extends along a full wall and our mirror cabinets do the same (they also extend to ceiling) Absolutely brilliant amount of storage space

I should add that they are recessed so don't stick out a great deal

Agree and give some thought about where your doors will split the mirror. You don’t want to be putting on makeup in front of the gap. You can also have open shelves on sides of mirror cabinet. Since you are having it made you can make it suit your needs and space.

Thanks for all of the advice and tips! :) We'll be going with (semi-) recessed as well

If you like it to be the same width as your vanity then do that. Most bathrooms dont have a lot of storage so shaving cabinets with plenty of storage for those little things is a big plus.

I think you can definitely go with same width and semi-recessed since you would like to have as much cabinet space as possible. We've had lots of customers getting same length vanities and mirror cabinets before for design or storage purposes, and their results look great. I've attached below photos from their projects for your reference ;). Same length can look great especially for rectangular mirror cabinets.

Thanks for the advice and all of the pictures - the bathrooms look beautiful!