I think size does matter, quite a lot infact, a bigger rucksack encourages you to take more and so a once roving approach, changes to a I can't be bothered as i have to much affair.
At the moment i am using a Billabong rucksack, which is great, it is meant for a skater, but it is brill, it has a very thin pocket internally, (Designed for a a4 pad) which is where i put licenses and day tickets, the rest is just empty space. It also has straps on the back (Meant for holding a skateboard) but are perfect for unhooking mat, jacket , landing net etc.
To top it all it is black, with a khaki highlights, (With lots of Mud) so it blends in where I fish
Ryan
(Make sure it is comfortable when heavy, well made and not too big)