Counter flashing length varies with pitch of the roof but no 'step' should be more than 3 bricks high. Width also varies but should always be wide enough to cover 4" of the base flashing. General rule of thumb in flashings is it should extend 4" over unlike materials (eg copper over/under asphalt shingles) and 3" over like material(eg copper over/under copper). In other words 'covering the bricks' with a counter flashing isn't the question (although more metal surface makes it vulnerable to wind damage). Rather the base flashing (which you can't quite see once job is done) is what does the major work in keeping water from entering house...... and roofers are notorious for reducing the 3" 4" rule down to 2" 3" as it lowers the material costs significantly. This information can be verified by the Copper Development Association and probably Nat'l Assoc. of Sheet Metal Contractors. Local library should have a few books on roofing and they generally offer the correct specs.