This question is tough, but depending on the purpose of the bag, it should fit everything you need in there. For example, if you are shopping for a Get Home bag, you need to acquire all the items you want in it first, then shop for the bag appropriately sized to fit all of those items. Buying the bag before the items may lead you to giving the first bag away to buy another one. It also needs to be as small as possible to carry all of your items. Make sure that once all the contents are put in the bag, that it is as light as possible. I have seen full Get Home bags weigh as much as my full Bug Out bag, and that should never be the case.