Re: how big are your grout lines?
All the marble floors I've ever seen or had installed had 1/8" or less grout lines. With natural stone floors, you want the beauty of the stone, not the blight of the grout. Grout is necessary to seal the joints, but you don't need big thick lines of it to accomplish that.
I say go with the narrowest grout line possible, if your stone setter can't do that, find one who will. As I mentioned, the tile companies that I've used over the years have had no problem with narrow lines.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!