I am assuming you are talking about the 12" polished marble, if so it is rectified, a fancy word meaning they are all cut to the same size, so you could use a 1/16" spacer or a 1/8" spacer. The larger the spacer the more room for adjustment. The rectified tile also have a chamfered edge, another fancy term meaning a slightly beveled edge. When grouting the chamfer gets filled by the grout so it makes the joint look wider than it really is, a 1/16" will look like a 1/8" after grouting. If a tile needs a slight adjustment replace the spacer at the point of adjustment with a toothpick and set it to the width needed to even things out.

As far as setting material goes, use a white colored modified thinset, HD sells one by Custom for marble and granite. Once you establish your layoot for the tile, set a ledger board all the way around the room for the bottom row of full tile to rest on, then go about putting up the tile. Are these tiles going in a shower also? And how high up the walls are you going, upto and include the ceiling?

Thanks for the reply! I want to put them up all the way - 8'. They are not going to go around the bathtub. Is there any problem of them falling off during istallation if I go too high?

They wont fall off the wall if you use a high quality modified thinset, but they are heavy so make sure the wallboard is properly secured to the studs.

no, they will not fall off the wall but they do slide.

I like to layout and work from the bottom up because of this and use tiny wood shims or toothpicks as spacers to keep the spacing consistent and so the weight of the marble doesnt crush the joints.

Marble tile sizing differenctials wil ldetermine hwo large of a grout joint spacing you will need.

the larger the tile size differential from tiole to tile, the larger joint you will need to keep the instllation running true and square.

I would try 1/8" and see if this wil lword by dry laying it out on the floor first. If it works there it will work on the wall.

The Tile council of North America and The marble Institute of America has some good literature.

http://www.tileusa.com/

http://www.marble-institute.com/

http://obscurity.ws/floorsearch.php?query=marble+tile+installation+instructions

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Thanks again, I will let you know how it turns out and post some pictures.