laying 14 x 14 to cement floor with thinset what size trowel should i be using i was told to use a round not square notched.

The trowel size is a factor of how flat your tile, and your tiling surface is, not the size of your tile. Generally speaking you should use the smallest square notch trowel that will provide 85% plus coverage. The flatter your surfaces, the smaller notch you can use.

All well and good if you have several trowels to test coverage with. If you have no trowels to begin with, then you should err on the larger size. the 3/8" x 1/4" notch seems to be the most common trowel used, and if there aren't deep cavities on the back of the tile, that size should work. You could buy 3 sizes, one larger and one smaller, and bring back the 2 that don't work. I'm sure a limited test with a trowel won't render it unreturnable.

Maybe someone wiser than I can post some pictures that demonstrate good coverage.

the reason they want you to use a round notch is because it is suppose to have better coverage on the bottom of the tile. You will use a little more thin set too.

If you got a flat tile and a flat surface, you can get the same coverage with all sizes of trowels, you will just have a thicker bed under the thicker notched trowel.

All depends on the backing of the tile too! I have seen very deep honery Comb grid on backs of tile that require a larger notch trowel.

That was " Honey Comb". My typing<<

Sure ...