As far as the tile size or pattern making a room look larger, I just dont see it, but it could be just me. A 5x5 room with a vanity and toilet in it wont look any bigger in my eye than what it is. Go with what looks good to you and what pleases you. Big tiles are in, but so are the mosaics. a 12" tile will be fine for all the rooms. The larger ones will be ok as long as the floor is flat enough to handle them. That is the issue with larger tile, if the floor has ridges and bumps they become more pronounced with a larger tile which cant conform to the variations in the floor as easily as a 12" or smaller. If the floor is flat and you can use the 18" tile go for it, its less grout lines to see. As long as the diagonal pattern does not cut off too much of the tile so that you lose the look of the pattern, do that too. Sometimes you can mix two sizes of tile, like cutting straight strips of the 18" in against the walls of the hallway and setting the 12" on the diagonal inside the strips. Or do the big ones on the diagonal in the kitchen and use the 12" in a brick pattern down the hallway.

it's really a matter of preference.

I recommend large format tiles for diagonal so you arent stuck with little pieces that you often get with small format tiles on diagonals.

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