The smaller the groutline, the better.

Kathy,. I would definitely do a 1/16 grout line - never shows dirt, and will never do any larger again! You will love it!

Grout color should be a tick lighter than the tile (I am not a fan of darker grout than the tile, but that is just my opinion). Your contractor should give you little sticks of grout samples to choose from. I think a good rule of thumb is to match your tile up to one that is the same color and go one shade lighter than that! Good luck and I like your tile choice :)

From what I understand from our contractor the tile dictates the grout size. I should add that because your tiles are larger and they are going on the floor, you may want to consider a 1/8" grout line. If we were talking about a uniform subway tile back splash - 1/16" grout line works (and I think are more preferable on vertical surfaces). 1/16" seems lean for that tile size. If your tile has variation even by a minute amount you will want a larger grout line. I am sure others will say 1/16" is doable, but it makes me nervous because you are depending so much on the skill level of your installer. I have 12 x 24 in my home and I did 3/16" and am very happy with them and feel like it looks very appropriate with the size of the tile.

I agree, the skinnier the grout lines, the better. And if you can, leave off deciding on grout until the contractor has laid at least some of the tiles, as the tile/grout combination will look totally different depending on the light and surrounding fixtures and fittings, etc. I think in your case I'd go for a slightly lighter grout, but not much lighter, because lighter shows more dirt!