Porcelain is fired to a higher temperature than ceramic so is therefore stronger and denser so is good choice for high traffic areas like kitchens and hallways.

No, wall tiles are not strong enough to be used on the floor but floor tiles can be used on the wall.

No this is a common misconseption, small tiles add more grout lines so can make the room look smaller, we recommend 300 x 600mm as a good size for small areas.