OK so the Surly Porteur House bag is pretty much the ultimate - it's huge, has heaps of compartments and looks awesome. It's designed to fit the Surly 24 Pack rack, but because it uses a system of eight Velcro straps to secure itself, you could attach it to pretty much any similar rack. Made from durable Nylon canvas with polyurethane coating to protect against the elements, it has a stiff insert at the bottom to give it shape and protect your stuff from wheel spray. Also, included are three urethane-coated, water-resistant nylon stuff sacks for an added layer of protection in each compartment.
The dimensions are 400 x 270 x 450mm, fully extended.