Solid white solid surface would look great and outperform anything else.

Any thing goes ;) I'd go to the bottom of the mirror. If you do a side splash on the left side, I'd probably have it just as high. You could do shorter on the side but it might look odd. Take white paper and mock up an area on the wall where there would be tile. Try a variety of height, widths, even draw tiles if you think it will help with your vision. When I did my kitchen and bathroom, I drew, or painted, cabinet ideas on the old walls before tear out.

We did not tile the side walls in any of our bathrooms because we aren't overly fond of that look, though I have seen bathrooms here and elsewhere with tiled side-splashes that look terrific. We made sure to use good quality, scrubbable paint in our bathrooms (we used BM Aura Bath and Spa paint; matte finish), and it seems to be holding up well*.

Over the sink in the guest bathroom, we used our border tiles from the shower as a backsplash (3" high?). In our MB and hall bathroom, we tiled up to the ceilings--that decision was more for aesthetics than necessity.

*I did end up scrubbing and touching-up (re-rolled) one small section of the wall between the vanity and the doorway molding in our MB after an unnoticed splatter of benzoyl peroxide gel/cream left permanent light colored spots on the wall. The section of wall next the sink/counter has been fine.

the big box stores carry 4" backsplashes and side splashes cut to size to coordinate with the solid surface or granite vanity tops. That would be an easy to install, practical solution.
I love subway tile, and would probably do two rows if I was doing the subway. But in my son's bathroom, I would use the solid surface or granite pieces. He won't care and I try to avoid spending time in there, LOL!

For a teen boy's bathroom I would have tiled to the bottom of the mirror and wrapped it all the way around from the left side of the vanity to a point past the toilet.

I plan on doing this when I remodel the kids' bathrooms, also boys. There will be tile wrapped all around the sinks and toilets! Much easier to clean tile than to have to clean walls and retouch the paint quite often.