just bought a new tail bag from an Italian company Givi. i was looking for something waterproof and BIG. this is the biggest i could find on the internet. to give you some scope, this bag is 80 liters and the next biggest i could find from any manufacturer was 63 liters. it's a roll top bag so you load your gear in from the top, velcro the seam at the top, then roll it over a couple times, then snap the buckles on each end to secure it and that's how your stuff stays dry.
this will almost eliminate my need for saddlebags, but you still have room for bags as you can see in the photo where it hangs over the edge. in this photo, i packed the following: folding chair, sleeping bag, pillow, a few changes of clothes, football, bottle of brandy, some knick knacks, and managed to snuggle in a 12-pack after these photos were taken. my only caveat so far would be that i wish it had a small external pocket on the outside somewhere like most bags do so you don't have to open the whole thing to get out a small item. and it does come with 2 bungee cords, but i used 2 extra just in case. if you don't have the bag filled very much, you might think of doing the same unless you criss cross the bungee cords like i did after taking these photos.
$129 shipped, ebay
29.9"L x 13.8"W x 12.6"H
those are the technical dimensions, but when the bag is empty and stretched up, the height is actually about 2 feet tall.