I would decide on what is available as target species. I have a plano guide series tackle box that I take everywhere. The main things I have I would say to always stock up on is your terminal tackle (hooks, weights, slip sinker stoppers, swivels). Next I would add the lures, baits, etc.. that you know will be used. If you never plan on fishing for a species don't bother with the waist of space.
Add a pair of fingernail clippers to cut line, needle nose pliers. If you use bug spray avoid anything with deet in it. It is the biggest fish repellent out there.
How small of a box are we talking to? I highly recommend the plano 4600 box. http://www.planomolding.com/fishing/fishing-products/
It isn't to much larger then a normal tray box, but it holds a ton more. I actually a huge chunk of my tackle into 2 of these boxes. Another option that I prefer is to condense down your tackle to be specific to what you plan on fishing for that day and add it into a backpack. I carry some survival/firstaid gear with me whenever I go out fishing (you never know)