Fire Brick is a standard size worldwide - the most common size being 9" x 4.5" x 2.5".
BUT… Standard / common bricks (from HD, Lowes, Rona, Menards, Mom & Pop’s…), they are ALL different sizes. Tthere is no “set size” for standard / common bricks… so we couldn’t put a set size in the instructions.
Every brick manufacturer has their own molds w/ their own unique brick sizes. And there are MANY sizes out there!!
Just make sure you order about 8% - 10% more than you will need so you won’t run out… the store runs out… color changes / slurry mix, etc…