Kill bags are available in a range of sizes. Two factors play into deciding what size bag to select.
First, your targeted fish species should be the primary guide in what size bag you need. A 40-inch pike or a 40-inch snook isn’t going to fit in a 30-inch bag. If you’re targeting panfish or mangrove snapper, you probably do not need a 60-inch bag (unless you plan to keep a lot of fish).
Second, consider the space in which you will use the bag. On the one hand, smaller bags are more efficient in smaller spaces. A smaller bag will be much more useful on a kayak than a bag that takes up most of your deck space and gets in your way. If you’re fishing from a boat and your deck can accommodate a larger bag efficiently, then larger bags are a great option.
Consider the weight of a full bag, as well, and how far you have to carry it. Dragging a 60-inch bag along a riverbank or beach might be less efficient than carrying a 30-inch or 36-inch bag on a shoulder strap.
Keep in mind, too, that one of the benefits of a kill bag over a cooler is that when you’re not using it (that is, before you catch the first fish), you can fold it up and store it more easily than you can a cooler. So, it requires less out-of-use storage space.