There is no hard fast rule on this. In my opinion, larger tiles make a room look smaller and small tiles make a room look bigger. However, 12 inch tiles are very popular and would look great in your 7 x 5 bathroom. Smaller tiles would just mean more grout to maintain!

Large tile in a small room, can work wonders in making the room feel larger than it really is, while small tiles in a sprawling kitchen will appear busy and overwhelming in such a large space. It's important that you take into account these practical considerations when choosing tile, and not just your first impressions in the showroom, to ensure you'll be happy with the final product.