Forgot to add: subfloor is plywood, since we were planning on using Ditra...

The issue at hand (or under foot) if that of properly supporting the tile.

If you look at a cross-section of Ditra, it's a waffle-type of structure. Once the tile is installed, the dovetailed depressions in the top side of the Dirta will be fully filled with thinset. But the voids underneath, those that are under what I call the raised ribs, they will be empty. They will be filled with "air".

{{gwi:1502886}}

Theoretically. And I'll write, again...theoretically...you want the tiles to be large enough to span any air-filled rib structures so that each end of the tile, or at least a good portion of each tile, is properly supported by the thinset-filled dovetailed depressions. That's why Schluter has the restriction for a minimum tile size that can go over Ditra.

Worst case: Someone with stiletto heels is standing on your 1" mosaic-over-Ditra floor. The high heel could place a point load on a tile that is supported on one edge by a thinset filled depression, with the other edge of the tile being over an air-filled rib. If the heel is over the air-filled rib edge of the tile, the tile could collapse the air-filled rib and pop down into the Ditra.

It's more of a liability in a downstairs public room where these types of point loads might be realized.

In a master bathroom? If you go barefoot or walk around in socks, no. If you wear stilettos while showering or brushing your teeth...maybe.

So it's a worst-case scenario.