The size is dependant upon what you would like to use it for. If it’s a small cooking surface then obviously smaller sized bricks (around 300mm x 200mm) would be better suited whereas larger ones (400-500x200 or even 600x300 mm) could work well for grills depending on how wide your oven doors open up. Just make sure that the size you choose is big enough to hold whatever it is your cooking on top of it.