We're redoing our small main bath and plan on light gray grout with white subways. You didn't say what style bath you are planning on. Gray grout tends to yield a more old-fashioned look, white grout a more modern look. (Though I've seen photos of gray grout with modern fixtures and of course vice-versa.) If you look on the web for white baths there are photos of white subways with gray grout, including in small bathrooms. I don't think the room size matters. Can't say anything about that new miracle grout -- our tile person hasn't mentioned it! We did our other bath with white grout a few years ago and there is already some yellowing, to our annoyance. Our current very old bath has tiny dark gray grout lines, and there is no issue of them clashing with the wall color. I don't think this will be an issue with light gray grout -- at least I hope not!. Of course if you pick a colored grout it could constrain future wall color options, if that matters.

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 with the rest. If it were a checkerboard, that would be a different story. Black grout would be the thing-- either way-- black or white would look exactly the same, believe it or not. Black would be easier to keep up. but being that you have white tiles against white tiles, the black would be TOO much of a contrast. Go with a medium grey, something like Custom Delorean grey, or Mapei Pewter.

I would door a darker grout to hide the grout lines so it looks more seamless.