Tile size variations

I am wanting to get part of my home tiled. It's about 1000 sf. I have chosen a porcelain ceramic tile, Continental Slate by Dal-Tile. The pattern is a "modified hopscotch" using 4 different sizes of tile - 18x18, 12x18, 12x12, and 6x6. The manufacturer's recommended spacing is 3/16".
As the installer started laying the tiles, he found that the pattern was becoming more and more "off", and he was having to make the grout lines wider around the 18x18 tiles to attempt to compensate. Finally he stopped working, as it became clear that it was getting worse, with some of the lines being twice as thick (6/16") as they were supposed to be.
Upon inspection of the tiles, it was found that all the sizes varied somewhat (which is reasonable - I am not expecting them to be all perfectly the same, but it would be nice), but in particular the 18x18s were the furthest off, ranging from 17 1/2" to 17 11/16", when they should be approx. 17 13/16".
How can these tiles be laid to compensate for these size variations? Should the installer be using several different spacer sizes and varying the grout lines?
Any help appreciated!
Thanks, Dee
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In each box, the tiles are probably the same. They may differ between boxes, however. Check the lot numbers. If all else fails, open all the boxes & take one tile from each box in sequence. Worked for me.

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If it is to far off dont use them. Home Depot sold my installer a bunch of bad tile and actualy paid for a redo
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Dee writes:

Any physical item varies in size. The question is how much. Dal-Tile should be able to provide specifications.
It is kindergarten tile setting to deal with size variations. If it is within specs, you accommodate it. If not, you send it back.
Your installer sounds incompetent if he doesn't know this.
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