Don't think about a first-aid kit as a fixed and immutable object. The kit should expand and contract, depending on where you're going, what you're likely to encounter, and how long will it take to get to a doctor, or have a doctor get to you. I have two first-aid kits, one for hunts, which are in close proximity to civilization, and the other for places like Alaska, where you may be left to your own devices if something happens. This saves me the trouble of re-building a single first-aid kit over and over. Whatever you take, it should be small enough that you can keep it with you. My small kit is a little bigger than a fist and fits in a fanny pack with no trouble. The larger one is the size of maybe two fists. Package it in something waterproof, like a Ziplock bag, or better, the rollup Velcro-seal plastic bags sold in camping-supply stores.