Here's where a mockup will help. I don't know what your vanity or medicine cabinet look like, so I picked a vanity that is 36' wide and mocked up a 30" wide medicine cabinet above it. Then I added in the light fixtures, which I also converted to scale. The background color is just there to make it easier to see the lights. It looks to me that the light that's closer in size to the cabinet gives a better balance to the whole. With Edison: With Weston:

just guessing here - but if the same principles apply to vanities that apply to tables - the width of the light should be roughly half the vanity width? Perhaps a triple sconce for the 48. For a long vanity, I'd probably do two double sconces - so say, 60 divided by 2, so 30, so each sconce roughly 15". Centered over each sink. Of course I'm guessing, and this is without seeing pics. And I don't think it should be much smaller than half, but larger would be OK IMO. (we have a double sconce over a small pedestal sink and it's fine.) But maybe that could give you a starting point...

I prefer end drawers on a vanity that long, making the side splash less critical and giving the person at that sink elbow room. So you do need a side splash to prevent water from going down the wall when the sink is close to a wall. Also, the counter edge may not be straight and tight to the wall, and a side splash covers the gap/uneven cut. Side splashes made with the 3 cm counter material look very chunky. If 2 cm is available, I'd use that. Or I'd use an ogee edge profile to minimize the blockiness. Personally, I think all that Caesarstone in the adjacent tub and large vanity area may make it look too plain and I'd use tile on both areas to add interest.

clear glass with Edison style light bulbs will give you either not enough light or an orange colored light. This is why I tend toward a frosted glass, so you can put the light bulb you like in it. IDK what stage you are at but I would tape out the mirror size you like and see where that would land sconces. What about doing a metal-framed mirror in a copper or ORB finish? I lean toward circular for some reason,