It is mostly a matter of personal preference. Standard plastic spacers are available from tile supply stores to place at the corners of tiles when you lay them. Remove the spacers before grouting. I personally think 6x6 tiles look good with 1/8-inch spaces for grout.
How Do You Decide How Much Grout Space To Leave Between Wall Tiles That Do Not Have Spacers? The Tiles Are 6" X 6" Non Glazed.
Normally, whatever the tile thickness is the width of grout line (i.e. 1/2 thick tile 1/2" grout line) spacers can be purchased at lowes.