It depends upon the material. While the size of the stone is a personal choice the sizes of the tiles should be proportionate to the size of the room - or the visible area of the room. The smaller the size of the room, the smaller the size of the tiles should be. This helps in bringing out the colors and look of the stone.
Granite is Natural Stone and Corian is a man made material with a plastic base. Granite is more durable and easier to maintain, as it is relatively scratch resistance compared to Corians, which are more prone to stains.
A combination of different sizes can be placed together to make a pattern. This adds to the beauty and uniqueness of the stone installed. Of late the use of patterns has become trendy.