A smaller tile will accept more of a slope. I agree with Henry in that basements generally (though not always) slope towards drains. Judging from the angle of your slope I would say an 8" tile with a 3/8" - 1/2" thinset mortar bed should do the job. maybe not as attractive as a 12" or larger tile, however, you have to work with what you have.
If the floor is evenly sloped and relatively flat with no humps or dips in the field area I would say that it could be tiled with a larger tile. It wouldn't be level but your tile installation would be flat.
Another more costly solution would be to have your tile installed like mud set marble and stone. This methods begins with the first tile being installed using a portland cement mixture (like sand topping) on the highest elevated point of the substrate. It is installed perfectly level and all subsequent tiles are installed in the same fashion running from the first. The end result is a perfectly level floor.