I'm also a fan of polished vitrified porcelain, have installed them in a family property of aroundabout your dimensions, and will be using them in an upcoming large development.
Pinkboys first post was on the money, cant fault anything he said. I would emphasis what he said about the floor needing to be level - if your tiler says that it needs floor leveler, take his advice and do it. The last thing you want is your tiler using very expensive glue to pack the level of the floor up.
As for them being slippery in kitchens, to be honest I can't say this has been the case in my experience, having lived in two separate homes with those tiles in the kitchen. The only time I ever slipped over on them was when installing a door - sawdust and polished vitrified tiles are a recipe for abruptly finding yourself on the floor wondering what the heck happened
In regards to tile sizes, it has been my experience that so long as you use a thin grout line, tile sizes and orientations don't really matter much because those sorts of tiles are so flat and regular that the overall surface tends to blend together, making grout lines less visible (and important) than in the case of a wide grout line and ceramic tile. In my opinion, using many smaller rectangular tiles is just going to increase the chance of the tiler making a mistake, and you trimming your toenails every time you walk over that spot barefoot, because porcelain tiles are sharp and hard.
In addition to Pinkboys posts, I would recommend that you test whatever tile you buy before you even think about final purchase, because not all porcelain vitrified tiles are equal. To adequately test them, get a sample tile and make marks on it using anything you can think of - permanent marker, whiteboard marker, lipstick, tomato paste, lead pencil, coffee, food, kids markers, crayons, keys - and then try to remove the marks at certain intervals ie right away, after letting the marks sit for an hour, letting them sit for a day.
The reason for this is that porcelain is porous, and will absorb various compounds if not glazed properly or enough.
When you inspect whatever tile it is, look at the tile from the edge - you will notice that the surface (glazed) layer is of a certain thickness. In general, the thicker the surface layer, the better the tile.
The other thing to look out for is that the manufacturers of porcelain tiles, particularly the Chinese, are a sneaky bunch - what they have been doing lately is to produce a poor quality tile, and then to wax it so that it looks/feels slippery, giving the impression of good quality. The obvious problem is that after the wax has worn off after you clean the tiles a few times/walk all over them, the porous top layer in the tile becomes exposed and they start to mark heavily and look terrible - so when you do your testing as described above, make sure to scrub, say, half of the tile really really well with detergent/scourer so as to get rid of the wax and expose the long term surface of the tile prior to marking with all of your test compounds.
To give you an example of how bad some porcelain tiles are in terms of porosity, ask a tiler how many times they've installed porcelain tiles and made permanent stains using grout, tile glue, or with a drop of sweat.
Anyway, good luck
