For someone who knows what they're doing, there's a little more thought involved with reference to layout and execution, but once started, it's every bit as easy. If you look closely at the pattern, it's just a matter of stacking the pieces, each piece a half tile off from the last, first on their sides, and then on end. Piece of cake!! (It just LOOKS complicated!)

Many of the subway tiles come with built in spacers for a small grout line - though you have to do some math to check that you don't have a gap at the top by the upper cabinets and fudge each tile a bit (a very little bit) to make it come out even. Mine have about 1/16 to 1/8. The grout line looks a little bigger after it is grouted. Here is mine: https://www.houzz.com/photos/my-pics-work-in-progress-phvw-vp~57088087 

ajracine, after I thought about your question, are you saying to take the 2"x4" inch (that is on a 45 degree with the herringbone) but run them in a straight pattern along the edge to line it? That would work. You might sacrifice a sheet to see how it will look to you. As long as you are okay with a 2" wide line of tile running vertical. Those would not need to be "polished off/bull nosed" like I was talking about. In my earlier reply, I was envisioning "rounding off" the cut edge of your mosiac. Sorry about that. I think I now understand. I think I would still go with a shluter edge or a thin pencil liner tile (if it blended well with your mosaic herringbone.) The reason I say that is the border you would created with the 2x4 tiles will pull your eye to the trim line/herringbone joint line, and take away from your pretty herringbone pattern. (or that is what my mind is seeing.)

I have a small kitchen too and I am looking for a 3" by 12" tile, but I am open to whatever works best. I like the brick-like proportions of longer tiles. But honestly, any tile is a vast improvement over the fake travertine 6" by 6" tiles installed on point (diagonally) that are on the backsplash now.