The mirror you are considering should work. IMO, you could go wider, especially considering the sconces are not wide. I would suggest mocking up the mirror you are considering with tape, get a feel for where the sconces will bee placed (height will most likely look best in/near the center height of the final mirror placement. As far as width, either anchor towards mirror (perhaps 6" to edge of sconce from mirror edge), or center in remaining space 9 1/4" based on 30" mirror and 67" total width. Just don't skew it towards the wall as that generally Durant look or feel right. IMO, ideal would be 36"w x 42" high with the sconce configuration. The size you are looking at I think would be the smallest size that would still look decent. Good luck!
Q