since it's only half a pic we can see, then tile up to the window. and just jog it over to the vanity splash. normally you do 4-5". where is the mirror going? you can edge this tile w/metal profile edging or get the carrara pencil liners or trim molding.

BTW, the wall board under the window, is that regular drywall? you should have some sort of waterproof substrate to tile on. If it's drywall, cut it out and replace w/hardiboard, apply a waterproof membrane such as Redgard, let that dry, then you can tile. Make sure when you do the cement board to use appropriate tape and mortar for the seams before applying the Redgard.

Also, make sure they do not use mastic to adhere this tile to the walls.

Do what Beth says.

As above or expect MOLD and MILDEW .