"As a designer who works in the tile business, I can tell you that your rule about 4 x 4 tiles in the bathroom is false. No, we don't want to go over a 4 x 4 tile on the shower floor, because of issues with pitching the floor, but on the rest of the bathroom floor, working with tile over 4 x 4 is not only sound mechanically, but also design wise.
"When we place a tile on the diagonal in a small space, the eye is drawn outwards to the points, making the space seem a little larger...smaller tiles are ok on bathroom floors, but sometimes, people do not want to be bothered with grout lines, that can stain (even if sealed, grout gets darker with time in the traffic pattern, just a fact of life), and make the bathroom floor look dirty.
"If we are looking to do a period look in the bathroom, something Victorian, maybe, then smaller tiles are most acceptable, but not everyone has an old house, and the look may interrupt a more contemporary design in the bathroom. Please check with a number of sources before putting down the 'Rules' of tile design."
CommentsEven if you want to take on a DIY tile installation project, it is always possible to consult a tile designer. It does not always have to be a full design-and-install package. Independent tile stores usually have at least a "design desk" or someone on duty whose expertise is design.
As a paying customer, or even an implied paying customer, you can get valuable advice from these on-staff tile designers that can save you a ton of misery in the long-run. After all, tile is permanent.