3/16. More if hand made. Or you will regret it. No contrasting grout unless someone was paid to float the wall out super flat behind the tile.

I'm a sucker for small grout lines so would do 1/16 if I had the choice. That being said, recognize that grout size is determined by more than just opinion; tile size and quality are two of the most significant factors that determine your grout size. Lower quality tiles with inconsistent sizing and poorly rectified edges will mandate that you increase your grout line to make up for the shortcomings, for example. Be sure to have this discussion with your tiler before the job starts so you're both on the same page. The worst thing is for a tiler to agree to 1/16th spacers, but then the job looks subpar because the tile quality couldn't support such a narrow grout line.