lol. Autumn, love how you prefaced your question! I say it all the time, "if the look becomes dated it's easy enough to remove and redo!"

Your diamonds are the right size. doing an accent? I'd prob do the entire wall and up to the ceiling w/the diamonds.

once you mark off a square above the stove, put in your border liners, you're only going to have a very small space for your accents. it's going to be too compressed. do the whole wall with them.

example: this is the same size as your wall . they tiled up to ceiling w/their accent tile.

use the subway on the other wall.

here's example of properly done accent over the stove. plenty of room. you don't have this.

Now this one is similar to your set-up. will your hood look like this? When it sits up high like this it's much better for an accent piece. look at this, and then look at the ones below w/the steel hoods that sit lower. look at the difference.

these next two pic have accents that are squished and detract rather than accent.

this one is borderline ok, but it includes the accent tile along the subways.

this one is done improperly, as the design is far too small,,,