I would leave enough room so that when I turned on the faucet I was not at risk of splashing my hands onto the mirror. If the mirror is on a medicine cabinet, be sure the mirror, when opened, clears the faucet handle when "on" If neither one of these issues are pertinent, then I would be sure that there is room above the counter so the bottom edge is not exposed to water. We had a mounted wall to wall mirror above the vanity and it rested on the 4" backsplash. Over a few years the mirror delaminated near the faucet/sink wet area. The mirror would get wet and run down into the bottom space between the mirror and the top of the backsplash. It just got worse and worse over the years and because it was affixed to the wall, there wasn't anything to do but tolerate it until time to do a gut of the room. As far as width, that would be your personal preference. If the mirror is on a cabinet, will the door swing interfere with side lights?

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.

Is the vanity in an alcove? If so, mirror the entire alcove and have the glass company cut the mirror for mounted sconces on the mirror. You could also do this if there's no adjacent walls or one wall, and you may have to. You have 38" to fit a mirror and two sconces (countertop overhangs an inch or so). I would want at least 3" between the sconce shade and mirror and the sconce and the wall, so 3" + 3" + width of sconce. I have a 24" mirror over a 36" vanity, which is a fine size for the mirror, but that leaves only 7" for a sconce between the mirror and wall. If you used a smaller than 24" mirror, the sconces might be too close together. I read that 30" - 36" is the optimal spacing for sconces. I always draw this stuff up on graph paper to check spacing and proportions.

I did a tile border when I remodeled the master bathroom in my previous house. I used a glass "molding" from Oceanside Glass Tile to frame the mirror. I also had 3 sconces placed though the mirror. The mirror needs to be done last ... don't let anyone tell you otherwise. If you have to wait then that is what you have to do, to get a mirror that is custom cut perfectly for the space. I think your mirror placement is too high and won't be comfortable for most people. The mirror height also looks a bit squat for the space you have and the overall proportions. If your tile won't go to the wall, how do you plan to finish the raw tile edges? You will need a tile trim/liner or Schluter.