Big enough to carry everything you would need on a fishing trip.  This bag has 3x compartments; the one on the side has a big rubber mesh outer allowing you to pack your wet wading gear away, the middle section takes 2-3 standard rod tubes and the third section is a general one for clothing, accessories and anything else.

In addition, there is a flat side pocket which is ideal for a landing net, with a mesh pocket inside and a smaller zip pocket outside for travel documents, keys etc.

With traditional carry handles, that can act as compression straps when the bag is not full.  For walking distances, there is a shoulder strap making it easy and comfortable to carry on your back.  Zippers face inwards to act as security.