Well, first of all, studs are about 1-1/2" wide these days, so that means the stud bay width with studs 16" on center is about 14-1/2". Having said that, if you have the skill to remove some material from either or both sides, you don't need to worry about weakening the studs. However, I suggest you leave this task to a handyman or carpenter since -- no insult intended -- I suspect you don't have the right tools or skill set.

Hello - I'm looking to replace my recessed medicine cabinet. The current dimensions are 15.5" x 23" (entire mirror) with a recessed opening of 14" x 21.5". I'd like to stay with the same recessed opening if at all possible Most options out there are either too wide or too short. I guess if I have to cut into the wall and add new supports I could

@margimutch: "What about matching the trim with the color/finish/style of the vanity?" That's an interesting idea though the vanity is Fresca vanity whose gray oak finish I couldn't recreate. I could paint the trim a similar shade or I could just paint the trim the color of the wall as Nancy suggested.

Not sure if Houzz will let me post a link but I’ll try: https://www.rejuvenation.com/products/mendenhall-medicine-cabinet/ Or search for Rejuvenation medicine cabinet—they make several variations, and all but one are the same dimensions. (Wright is the one that’s different, I think.) We have two (one purchased for a new bathroom last year and the other for our existing bath 9+ years ago) and really like them; initially got one from another store and returned it because the quality was disappointing. We were DIYing the first installation and building it from scratch was well beyond our skillsets, but this was pretty simple. We are in a very high cost labor area so would have paid considerably more to have a carpenter build one. YMMV if you’re in a lower cost area.