A few suggestions... There's a Restoration Hardware outlet at the Tulalip Outlet mall so you might find a few deals there. ( I remember you mentioned living in Edmonds, so it might be worth the drive up north.) Or, as Babs suggested, having a glass and mirror shop custom make a nice beveled mirror is very reasonable. When we remodeled our master bath we made a frame with some nice trim/molding that we bought from our cabinet maker (to match our cabinets) and had a mirror with an extra large bevel to fit in the frame. For our guest bathroom I found a simple, wide, solid wood, oversized picture frame at Aaron Bros. on sale for $20, and a local glass shop put a beveled mirror in it for me for $40. It looks fabulous! (We used a metal bracket called a hangman which makes it easy and safe to hang large, heavy objects.) In regards to the size, personally I think bigger is better, especially vertically. I've always noticed nicer hotels have wonderfully large mirrors - they really add a feeling of spaciousness to what are usually small rooms.

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?

I think a 3-light will probably work. A 2-light in a powder room, maybe, but you'll want something like 3x65W equivalents of light to get ready in the morning. As long as it's not wider than your vanity, a little larger or smaller than the mirror is not likely to be noticeable.

So yes, according to the above, it would be.