I only see wall tile in rectagular shapes, all the floor tile I see is always square. Since she is already up for the extra costs of marble supplying her with rectangular floor tile in marble shouldn't be much of a problem at all. 20x20 marble isn't that unusual and the additional cost (depending on the square footage of the job) shouldn't be much to cut it down 20s to 9x18s or even better 10x20s, really just a matter of setting a fence on the saw and running them all through. Doing the same with ceramic however would be less than ideal since the cut lines on each tile would be noticeable vs the other factory edges. Those are my experiences with my tile suppliers, perhaps other parts of the country have some different experiences.
Yes, however watcch for beveled edges, unless you plan on cutting all 4 sides. Stones can be found without beveled edges, but with them also, so check samples otherwise the cut side will look a little different.
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