What should be the size of the stone you use for floors or for walls?
Answer: It depends on the surface area of the floors or walls in consideration.
Answer # 2: It also depends on how much do the eyes see in one go. Now that is a difficult thing to figure.
Answer # 3: It depends on how patient you are with a process of making a beautiful floor or a beautiful wall.
Generally speaking, smaller stones would serve better and look lovelier for a long period time.