If space wasn't an issue (of course this wasn't always the case), always bring the longest and widest sharpener you can carry. That said, the size of a sharpener also depends on the length and the blade shape of the knife/knives you're carrying. I prefer diamond coated sharpeners because they're easy to maintain, and you can use them wet or dry. For my size of knives, I always keep a 6"x2" DMT Fine in my backpack. Recently my favorite is the double sided DMT Diafold coarse and fine (about 4" long folded).
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Reynaert
Fayetteville, AR