Red Firebrick are typically stocked in 2 sizes:
1-1/4″ x 4″ x 8-1/4″ (Thin)
2-1/2″ x 4″ x 8-1/4″ (Full)
1-1/4″ x 4″ x 8-1/4″
2-1/2″ x 4″ x 8-1/4″