I think, screw that rule and get as wide a mirror as you are able. I'm planning to put a 48x30 mirror over my 56" vanity. So far I've found nothing already made that I like or in my budget. Went to my local glass shop and they will make a 1/4" thick mirror, 48x30, unfinished edges, no bevel for $90. To bevel all around (I forget how wide) it's $145 for the mirror. I bring in my own frame and they charge $20 to put the mirror in the frame. They said, the frame comes first. Makes sense. I can get whatever size frame I need via pictureframes.com for under $100.

I agree with Lotteryticket about a mirror not much wider than the sink. I'd aim for one where the glass is just the width of the sink or narrower. I'm not certain where/what mirrors you have in mind, but I see RH has mirrors that are 18" wide. I suspect that would look fine above a 16.5" pedestal sink. I'm not a big fan of having a mirror that spans above a toilet.

You don't have to match fixtures in different baths. In fact, you probably couldn't find a designer to recommend that. Builders do it because it is cheap and easy to put the same products everywhere and in every unit. As for the size, it is clearly a matter of personal taste. However, most women find it practical to have the light coming in at an angle rather than overhead.

Thats a very specific size you’re looking for, your options may be limited. In mu quick search, I didnt find one lighted mirror at 40x36. You may find some though. Go to some lighting store, they’ll likely be the only place to actually see one in person. General things to consider : -Look at reviews for the look or mirror you like. You’ll likely find out fast what is good and what is garbage. Normally you get what you pay for, but you can really pay a lot for some of these mirrors. -Surface mount or in-wall? If you want in-wall, make sure the ones you look at are rated for in-wall mounting. Surface mounts can stick out a fair amount from the wall. -What are your lighting requirements? Is it just for ambiance or general lighting, or do you need specific lighting for makeup or other tasks? That should help narrow down if you need a full surround light, side lighting, top/bottom, or back lighting. -I’d opt for something fairly bright, with a dimmer so you can adjust as needed. The brighter lights will also reduce your chances for wanting/needing an additional fixture. -Get something with a decent warranty, or lighting that is fairly easy to fix or replace. If the lighting fails in 5 years after your warranty is up, you have a very expensive mirror that you need to replace, or live with. -I would try to buy locally, if the mirror breaks during shipping, thats an akward box to deal with returns/exchanges if shipping.