That's pretty small if you mean overall length. If you mean a 5" or less length on the blade alone, that makes it easier to pick.
Easier, but not "easy". Many words have been written about "do it all knives". And while I won't tag Bill again cause it has been done twice already, he IS the man I ask about knives.
In my opinion though, is going to be tough to beat an ESEE 3 or an ESSE 4. It ain't the biggest knife out there nor is it the most expensive. But if you're going to put it on a plate carrier, I wouldn't want much bigger than that. It is big enough for field work and made well. As a young man I leaned toward the ESEE 4 but the older I get the more I like the ESEE 3. Smaller = a bit more controllable in my opinion. Goodness forbid you've have to do something that takes a little finesse. A smaller knife is great for that.
Knivesshipfree has some ESEE 3's in stock and they have very good customer service.
Shop our in-stock selection of ESEE Knives. We are a small business with big numbers of knives!
www.knivesshipfree.com
After you get that base covered, you can step into the world of Bark Rivers and Lon Humphreys. This is perhaps my favorite "do it all" knife.
It's the Rustic Bravo. The one with the thinner (relatively) scales is the one I'd grab if it all goes to crap. This one is about identical to mine.
If I had to fight though, and it didn't have to be a do it all knife, then I throw this one on. The Spartan Harsey Difensa.