18"x36" -perfect for small combos or head units24"x36" -for larger combos or speaker cabinets36"x36" -big enough for most 4x12 cabinets48"x36" -for those "taller" cabinets54"x36" -"taller" still72"x36" -will do a couple of typical 4x12 cabinets
The first number in the size (example 18" in 18x36) is the roll width and the second number is the roll length. Purchase a quantity of "2" you'll get a piece 72" long, if you purchase "3" you'll get 108" etc. Also, for proper orientation (particularly on Fender installations) the first number will be cabinet height, and the second number will be the cabinet width. This will keep the pattern oriented correctly.