According to everything I've read on this forum, it depends on what kind of tile you have. Ceramic tile should have a 3/16th" grout and porcelain can be as narrow as 1/16th".

Was your tile cut into 13 X 13 before or after the kiln process? Ceramic tile is not as uniform in size as porcelain but now I'm wondering if ceramic tiles even comes in 13 x 13 tiles.

I'd wait for the real experts to weigh in. I'm mainly writing what I think to see if I've got it right myself. Ha!

Ceramic tile should have a 3/16th" grout and porcelain can be as narrow as 1/16th".

Golddust, you're close. :-) Conventional ceramic and porcelain BOTH should have a minimum of 3/16" grout joint (which is what I normally use), due to sizing of the tile. Only if the porcelain is rectified (what you were talking about-- the tile being cut to size after firing) can you use the 1/16" grout joint. You got an "A" for effort, though! :-)

Best bet, if you've got tile questions you're not sure of, is to check the tile FAQ in the gallery side. Most questions can be answered there:

Here is a link that might be useful: Tile FAQ