For such large tiles, a 5mm grout width is recommended. This is because it helps alleviate substrate movement and stresses within the installation, helping protect the tiles.
For perimeter and movement joints, grout should not be used to fill these joints as it is not flexible enough. As larger tiles are more affected by movement you should use a movement joint where there is a joint in the sub-floor or change in the sub-floor type. You should also use them on heated floors where your tiled area exceeds 40m2 (with no one side exceeding 8m in length) and where your tiling exceeds 10x10m (with no side exceeding 10m in length).
Around the perimeter of the tiled area and internal corners and joints (where two adjoining walls meet or where a floor meets a wall) you should use a flexible joining material such as silicone sealant.