Your oven is 18" deep?
Mine is 17" deep as far as fitting a steel, but there is an extra inch where the window is. My steel is 17X18x0.5. I make an 18" pizza by letting the last inch hang over the edge of the steel. After a minute or so in the oven, and the crust has set, I lift up the folded over edge and rotate the pizza just a bit. The last 1" sticks out over the edge of the steel up into the area in front of the window. I do a couple turns over the next 4-5 minutes. 585F.