Also, consider the VAGX rolltop from here. The seller is on this forum, and made an appearance in another thread, when mooting his idea to sell those bags. Read from post 25 onwards.
Cavendish Cycles sell the Bagaboo range of bags, and in one photo, they even fit a young woman in one. There is still a bigger size.
There are other huge bags, but I'm sure you'll find them.
There are plenty of bags that I feel would suit you, but for cheapness and size, there is the Linnel's black messenger bag. This is a larger version of the normal red postie bag. Its huge.
Also, consider the VAGX rolltop from here. The seller is on this forum, and made an appearance in another thread, when mooting his idea to sell those bags. Read from post 25 onwards.
Cavendish Cycles sell the Bagaboo range of bags, and in one photo, they even fit a young woman in one. There is still a bigger size.
There are other huge bags, but I'm sure you'll find them.