No. A vanity mirror should match the width of the cabinet or stop slightly inside it, so the glass edges fall within the countertop edges rather than hanging past them. Wider reads as a mistake unless the mirror runs the full wall on purpose, which is a different design decision. Height and centering matter as much as width.
The eye treats the counter as the outer boundary of that whole assembly, so a mirror that overhangs it looks like something bought off a shelf and hung wherever it landed. Matching the vanity width is safe. Coming in an inch or two on each side is safer still, and it is what most designers do, because a hair of shadow at each edge makes the mirror look intentional.
Sconces take width out of the budget. If lights are flanking the mirror rather than sitting above it, the glass narrows to leave a clean, even gap on both sides, and that gap is decided before the mirror is cut, not after the electrician has left.
The exception is a mirror run wall to wall, sometimes from the backsplash all the way to the ceiling. It stops being an object and becomes a surface, so there is no edge left for the eye to compare against the vanity below, and the rule simply does not apply. It also doubles the apparent depth of a small bathroom, which is the reason it turns up so often in powder rooms.
What a wall to wall piece does not tolerate is a room that is out of square. The glass has to follow the corners it lands in, which is template work rather than a stock size.
Height is where most bathrooms go wrong. The bottom edge should clear the backsplash by a small margin, so splashing and toothpaste never reach the silvered edge. The top edge is best aligned with the head height of the door and window, which keeps the horizontal lines in the room agreeing with each other. And if the sink is not centered under the cabinet, center the mirror on the sink rather than the cabinet, because the sink is where a face ends up. Two people of very different heights sharing one bathroom is the usual argument for going taller, and height is the cheaper dimension to add, since it disturbs neither the sconces nor the counter line.
Both work, and the rest of the room decides. A polished or beveled edge with no frame keeps the wall quiet and lets tile and hardware carry the design. A frame introduces a line of its own, which rescues a plain bathroom and competes in a busy one. Frameless is also the easier piece to keep clean along the bottom.
One long piece should run close to the full counter length and stay inside its edges. Two mirrors should each be sized to their own sink with an even gap between them, wide enough to look deliberate. Sconces mounted between two sinks change those numbers, so the lighting and the mirrors get laid out together.
Send the vanity width, the ceiling height and where the sconces land, and the layout can be worked out from there. The Midland Park showroom on Greenwood Avenue runs by appointment and keeps glass and finish samples on hand if you would rather see edge work in person.
