I've seen two types of firebrick splits used to line wood stove fireboxes: high-density and high-refractory. Some manufacturers use smaller or cut-down bricks to fit their fireboxes, but the typical size used is 4.5" x 9" x 1-1/4". Pre-epa models used high-density bricks weighing 3.5 lbs. apiece, but most of the newer, epa approved models I've seen use high-refractory bricks, which are extremely porous, weigh only 2 lbs. apiece, and appear to have chunks of vermiculite or some similar material in them. The lightweight high-refractory firebricks do break down at a much faster rate than the heavier high-density bricks, but I've been told that high-refractory bricks make for hotter firebox temperatures, which in turn create better combustion efficiency and lower emissions. Within the average wood stove firebox, the bricks that take the big hit are the floor bricks, which are subjected to more abrasion from fuelwood and ash shovels than the side bricks. This is one of the reasons we advise our customers to always leave an inch of ash protecting the floor bricks.