If your old fire pack is the standard lumbar pack I've seen of around 1,300 cu in, it should make a great daypack - and if it's part of the modular Nimrod system, you can easily add their Wilderness Gear Bag (B200f) for another 3,400 cu in of space for about $140. That should be as big as anyone needs!

All that is necessary for the triumph of evil is that good men do nothing -- Edmund Burke