kohler archer 20"

Unless there is some near adjacent wall limit or lighting issue ? I'd make the med larger than the vanity and go up to 24 or 30 inches.

Helpful to see a picture, know the style of the bath, vanity etc

Robern has several option for width, height and depth. They have 12, 16, 20 and 24 available for many of their collections. Go with a 20" wide, but make it taller.

I have tall ceilings so I used two 20"W x 48"H medicine cabinets in my guest bathroom.

I have an upper cabinet inside the wall as a medicine cabinet.

Here we are :-))

https://www.homedepot.com/p/Glass-Warehouse-Calla-22-in-W-x-32-in-H-x-5-in-D-Recessed-Medicine-Cabinet-in-White-with-Mirror-SC1-SQ-22X32-W/321724457

https://www.wayfair.com/home-improvement/pdp/glass-warehouse-stainless-steel-recessed-framed-single-door-medicine-cabinet-with-adjustable-shelves-gaks5072.html

Robern makes varying sizes. You can probably order one on Build.com.

https://www.wayfair.com/home-improvement/pdp/glass-warehouse-recessed-framed-2-of-doors-medicine-cabinet-with-adjustable-shelves-gaks5075.html