The tile type often dictates what size joint space to use. It is desirable to have small joints in most cases. If a tile is rectified a 2 mm or 3 mm space is possible. For many floor tiles a 3 mm, 4 mm or 5 mm joint is good. It often depends on the calibration of the tiles. Unless a tile has been rectified, most tiles are not 100% perfectly sized from one to the next, therefore a bigger joint than 2 mm is required to enable a proper installation. They can sometimes vary in size from tile to tile by a half millimeter or so due to baking and drying factors during production, which is normal.