The Raimondi Tile Leveling System will make sure your new large format tile will be kept level as the thin-set dries overnight or for the next 24 hours.
The leveling system has 2 parts - clips and wedges - and the wedges are reusable while they last. Also needed is Raimondi Pliers which any tile installer that uses this system should have.
See these products on Amazon in the links below:
Raimondi Leveling Clips
Raimondi Leveling Wedges
Raimondi Leveling Pliers
Please see the very important explanation below to help decide what size clips to use.
(This page was quickly posted just to explain the process but it'll soon be updated to show everything in pictures for a better understanding.)
The Raimondi clips come in different sizes and you decide what size you need based on the grout width you want. So in theory, 1/8 in. clips should keep a grout line of 1/8 between your tile as long as your tile's edges (where the grout goes) are perfectly vertical and parallel like this... || ...and do not form this shape... \ / ... which is more like a V.
See, the clip goes at the bottom of the tile and that's where it sets the distance in between 2 tiles. So if your tiles have edges that go down like this in the grout line... \ / ... then the bottom of that shape will be 1/8 and the top can be quite wider - and remember, that top is the grout width you'll see when the tille job is done.
So before you decide what size clips to use, check your tile like this... Take 2 tiles and put them on a (protected) table or other hard, flat surface and carefully push them together so they touch each other on the long side. CAREFUL: use eye protection and gloves as you do this because it is easy to chip a corner if you touch/bump the tile into one another the wrong way and sharp pieces of tile can hurt you real bad.
Once the tile are touching on the whole length, look either from above or from either end of the imaginary grout line and see if there is a gap in between the tile that looks more like a V. If you see more like a V, than the bottoms of the tiles are touching and the surface areas are not. The Raimondi Clips will increase the gap at the bottom from the current zero to 1/8 (if you're using 1/8 clips) and the top surface of the future grout will be whatever you see now plus 1/8 (or whatever clip size you'll use).
If the 2 tiles are perfectly touching and there's not much gap in between them and especially nothing like a V, then you should be almost certain that when clips will be installed the right way in between those tiles, the width of the clip you chose should be about the same as the final grout width you want to have.
Hope the above helps in deciding what Raimondi Leveling Clips size to use, while the Raimondi Leveling Wedges are not affecting grout line width. And your chosen tile installer should have the Raimondi Leveling Pliers is they use this system.