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 can't give unbiased advice because it sounds like we are doing exactly what you are doing. I think it's a personal preference thing. We are tiling 12 inches above vanity top (to the bottom of the mirror) and doing the whole wall behind vanity and toilet right to shower which will be tiled in the same tile. This is how we had it before the reno and I liked having tile behind everything - for both ease of cleaning and the look of it. The sales guy from the tile shop said that most people weren't tiling whole bathroom walls these days, but I have a classic, old world aesthetic (not necessarily traditional) and his clientele is mostly the Mcmansion or new build owners. Like I said, personal preference.

This is a picture of the top of the vanity and 5" depth and space above the vanity mirror that we have. Thanks!

Thanks for all the replies. J Design - the ceiling is 8 feet high. To the left of the photo (unpictured), there will be a new 60x30x18 tub. The back wall, pictured, is 80" including where the tub is. That leaves 50" and realistically 48" to play with if I space the edge of the sink at least 2 inches away from the tub. After looking more closely, I'll need a pedestal sink narrower than 22", the sink pictured is 19" and the tub now is 29" on the most outer part. ACM, I think unfortunately, having even a small vanity would seriously crowd the tub, or vice versa. We hope to put a partial glass enclosure but need to think about if we can hinge it at a place that still gives us room to exit the shower near the sink. Does this all make sense?