Since you have a wall mount sink, most of the floor will be visible. Go with what you love. The 12x24 will look great and will give you the modern look you're going for.

If your quartz countertop is solid or just has flecks of sparkle, it's nice to bring some pattern in with the floor tile. You can also add some pattern to the floor with the way you lay the tile. Try to find a tile store that also sells quartz so you can coordinate the two. Be aware that you may have to add a new subfloor for the tile, which would make the tile higher than the hardwood in the hallway. This happened in our kitchen when we replaced sheet vinyl with porcelain tile. I do not like the wooden "speed bump" strip between the hardwood and tile. We would have liked to use porcelain tile in our guest bath remodel, but again, it would be higher than the adjacent carpet. We used Armstrong Alterna LVT. There aren't as many color and pattern choices as porcelain tile, but it it has a limestone composite base and is grouted just like real tile, so it's not really vinyl-y. Nice stuff, and a lot cheaper than porcelain tile.

Id think about doing a lot bigger mirror, so you can reflect the lighting you want to hang and mirror mount s small light over the vanity. Tho I do feel the light fixture on a hole might be a tad too large.

I'm also loving the moody blues but I agree with @jck910 in that the paint should blend with the floor. I was going to suggest SW white egret but I just watched a video on it and the video said if your room doesn't get a lot of light it will make the room feel dingy. I used it in a room with two sky lights so it wasn't. That said, I think something warm and white/creamy would be good just from looking at our picture. I would freshen up the trim and window sill with SW extra white in semi gloss. So many colors to choose from!