I also found vibration much less on my sunglasses than helmet.
I would go with the larger mirror.
I've seen some mirrors adjusted with the lengthwise vertical, but I've always had mine lengthwise horizontal. Someday I might try it the other way but I've been satisfied. I think mirrors can be a lifesaver especially since I ride on a road with lots of traffic. I do like to know when I'm king of the road or not.
Women here have given you great advice, just give your brain a chance to adapt.