What you want to do is cut your pieces about 3/16 shy of the measurement you want, and that would be your joint size. Any time you use an interlocking pattern like this, you need to take that grout joint size into account when deciding the exact size for your pieces. In this case, it would be 2 13/16x 5 13/16. If you want to get daring and make them tighter, whatever grout joint you decide on, subtract that size from what you want to be the modular size of your tile.

One other thing-- if you're going to cut these tiles up (as opposed to being factory sized) you want to make sure whatever size you cut them, there's not a single factory edge left on the pieces. Otherwise it'll show alot differently from the cut edges once grouted.

Bill, I was hoping that you would respond! Thank you. But I am confused.

I don't really have my heart set on any particular size tile. The only reason I am thinking about cutting the pieces smaller is because of the dramatic difference in the color of the tile (which is a limestone). I thought mixing up smaller pieces of tile would work better in making the stone color variation less dramatic. Is that true?

So given that my tile can be anything (unless you think it should match my 3 x 6 shower tile), do you think a herringbone pattern works better with tight or wider grout?

Also, the "seagrass" (that's what Walker Zanger calls it, although we got a less expensive version somewhere else--may be that's why we got such a mixed lot!), is a limestone, that came in a 12 x 12. Since it is a natural stone, it has very straight cut edges. When my tiler cut it in half (to make the 12 x 6 that I was originally planning to use) I could not tell the difference between the edge he cut and the factory edge. But I am a novice homebuilder. Am I missing something?

BTW, the really sharp edges I thought would allow me to make the grout lines tighter. (I just don't know if that looks better for the herringbone pattern.)

Thanks!

So given that my tile can be anything (unless you think it should match my 3 x 6 shower tile), do you think a herringbone pattern works better with tight or wider grout?

My personal taste is for 1/8" joints. it's just large enough that with a grout color that's slightly off from the color of the tile, it'll show the pattern up very nicely, without overwhelming the tile itself.

I could not tell the difference between the edge he cut and the factory edge. But I am a novice homebuilder. Am I missing something?

Not at all. Most natural stone tiles have a small chamfer on the edge, but some don't. You should be fine without trimming the factory edge.