IMO the 12 x24 are a bit big for backsplash you could use a smaller accent tile or the same tile s you are using on the shower floor. As for height I like a simple 6” and in you case because the sinks are close to the wall I would do the same to the edge of the vanity on both sides sort of this idea just use schluter though not the trim piece
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tile sonoma montage series 10 x 16 glossy chroma metallic 2 x 12 silk chrom Metallica
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