What I did in my kitchen was to tile a 9ft wall above my stove and a 7ft area above a desk. I bought cheap slate tiles from Home Depot and smashed them with a hammer.
I then tiled those areas with all the random pieces and just glued them to the wall. The problem was with the grouting. I used a putty colored grout which blended nicely with the grays and browns of the slate. But since slate is horribly uneven, it was really time consuming to get all the grout lines and uneven textures of the slate looking clean and professional. The tiling was a piece of cake but my hands were bleeding from grouting for a couple of days after that. It really ended looking extremely cool, I have to admit.
I tiled a very large floor many years ago and if I remember correctly, I think we used 1/4" spacers. The grouting was still the worst part. I think I went through a jar of hand cream after that.