All depends on the type of tile you have. NO ONE can answer with out know. Some require a sanded grout so they need to spaced at least 1/8". Some require a unsanded grout and can be butted almost together .

This has to do with the tile body if its a soft bodied or hard bodied and the water absobtion.

Any questions you can e mail me through my avatar and check my qualifications there. GL..

The tiles should be touching each other.

These large glass tiles placed in rows looks amazing.http://www.modwalls.com/4x12subway.aspx