I love those long floorboards/wood-look tiles and they are the feature so you wouldn't want something too busy on the walls... personally I would go with a larger format say 300x600 rectified edge white tile with a matt finish for the Hamptons look... what's your vanity?... a timber one would tie in nicely with the floors and all the white... very Hamptons :)

We're building 4x3.5 Two with ensuite and the others with a jack and Jill. Life has changed unfortunately and now everyone in the household works, kids stay at home longer and ageing parents sometimes move in for care. At the moment we have a 3 x 1 and really the only thing we really want for is a second toilet, which growing up we had because we had the one out the back with the red backs.

How big are the floor tiles? It may look unbalanced with a more delicate tile in the floor and the 600x300 on the wall. Are you able to continue the flooring tile up at least one wall for cohesiveness.

I would prefer to keep the WC separate to the Bathroom as you have 3-4 kids? who will get bigger very quickly, having a powder room downstairs won't help with the morning rush to school., But it's your house so I have added a pocket door between the WC and Bathroom. I have also taken that small robe space for an extra vanity in the WC/Powder room. No width dimesion given for the WC The ensuite could use that extra entrance space as a sit down makeup and hair bench for your wife.