The space between the tiles should be uniform. Use spacers if your tiles don't come on mesh sheets. The larger the tile, the larger the space between them should be. Some do-it-yourselfers will make the mistake of pushing tiles too close together to reduce grout lines. Without enough surface area, grout won't bond well and can fail prematurely, leaving room for leaks and water damage. It's also very important to let the adhesive cure fully.