FWIW, the industry guidelines for grout joints is a minimum of 3x the difference between the shortest and longest dimensions.
On a backsplash, you might want to consider an epoxy or an acrylic grout as they are less susceptible to stains if that might be a consideration. They're not impervious, but likely better than a cement based one.
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Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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