generally, standard inset medicine cabinets fit in the stud cavity (between two wall studs)

wall mounted cabinets are mounted on the finished wall...

you can have a custom built, wall mounted cabinet to you specifications.

You are use to a lot of mirror, it will be a change, for sure.

Decide what you need in storage before you make a decision.

Minimize is the word. As shallow as possible. Again minimize

Do you even need one if your vanity has drawers?

Are there two sinks in your vanity? I know it's hard to get good pictures in bathrooms sometimes because there isn't a lot of room. I like to have sconce lighting in a bathroom, meaning lights on either side of the mirrors instead of overhead. It's better quality lighting for grooming. Is (are) the sink(s) going to be in exactly the same position relative to the edges of the vanity? I would choose skinny light fixtures since the vanity butts up against a wall, then measure to see how much room you will have for a medicine cabinet after placing sconces. While they are meant to be placed in stud cavities, when you open the wall if the cavity isn't in the right place you can still make it work. When we remodeled our master bath not too long ago DH had to move the cavities because of a design change from old to new. If you aren't DIYing your contractor should be able to do this easily. Medicine cabinets come in certain standard sizes, so once you figure out which size you have room for you can choose one. If your ceilings are 8' I think 30" tall is enough. If 9', you can easily use a 36" one. I don't know if there's any value to a taller once since at that point the upper shelf might be hard for shorter people to utilize well. Place them so the bottom of the door won't interfere with the faucet when the door is open.

IMO the easiest solution is to purchase medicine cabinets designed to be inserted between studs and install them at either end of the vanity. Can you ever have enough storage for medications etc.

IMO big enough to hold whtat you need and as for fitting between studs all you need is to build a header lust like for a window if you want bigger. We do not actually have medicin cabinet we have drawers to hold all the items we would put in a wall cabinet any stuff that would be dangerous is in our bedroom since we have a small great grandson . IMO a wall of mirror is a great choice and you have lots of drawers so use them . I keep extra TP in the vanity cabinet under the sink

Definitely looking for medicine cabinet vs. mirror wall. This will be a 60” vanity with one centered sink. A lighting fixture will go above not on the sides. Cabinet will be surface mount. Contractor has suggested a 48” medicine cabinet; that seems big to me. But will a narrower cabinet (30 or 40”) look too small?