I use tiles that are 6 and 1/2 inches square and about 1/2 inch thick. Cover the upper shelf wall to wall and front to back. My old GE electric stove takes 6 tiles. They last quite awhile...sometimes one will crack but they are still usable. I recomend taking them out during cleaning cycle. Properly made ciabatta has enough moisture (75 to 80%) not to need water in the oven or spray on the walls. For other rustic breads I pour hot water into a cast iron skillet on the bottom shelf. Carefull with water on the glass door of the oven...it is expensive to replace.
