Done it loads of times - its solid even without any weight on the foam you cannot drag it across the tiles to re position it. But my tiles are weathered concrete and the OPs look like the sand textured ones, I would not try it on slates though.
A good size and thick ness would be about the size of a babies cot mattress the foam also protects the tiles.
I had a relative throwing out a foam mattress and I cut it up into useful pieces.
Out of some I cut it into a wedge shape and added a wooden top with contact spray adheasive and so had a stable level platform on a pitched roof to hold a bucket mortar for repairs.
