How can I replace a medicine cabinet with just a oversized mirror?

Hi Sylvia, even if your medicine chest is inset, you can remove it and place the new mirror or larger new mirrored medicine chest over the old hole in the wall, I replaced a small medicine inset with a large 3 door mirrored medicine cabinet

https://www.lowes.ca/medicine-cabinets/style-selections-3625-in-x-2975-in-white-particleboard-surface-mount-medicine-cabinet_g1190410.html

Remove the old medicine cabinet and then if you want, here is an easy tutorial on how to patch the hole: https://www.hunker.com/12454882/how-to-cover-up-a-hole-left-by-a-medicine-cabinet

If you don't want to patch the hole, just hang the mirror over the hole.

I am horribly lazy. I would take the medicine cabinet down and just put the mirror over the hole that is left. Would look nice if you framed the mirror after putting it up. We did that and it turned our cheap looking builder grade mirrors into something that actually looks nice. We just cut trim to fit and tacked it in place. Made more of a difference that I initially thought it would.

You'll want to get spackle and a piece of drywall to close the patch once you take the medicine cabinet out. Otherwise, you can just hang the mirror over the patch if it will hide it and you're okay with that.

You can. I have done this. If the cabinet is built in then you should just be able to take the door off and the hinges. Then hang your mirror over the spot. Otherwise it should unscrew from the wall and you can remove it. If it is a bigger mirror make sure to hang it in the studs. You can use a stud finder to locate them. Good Luck!