DMT puts out a lot of good pocket options, their card size sharpener, mini's, diafolds, etc. So check out their website and see what interests you. If I had to pick just 1 grit, it be Fine, second choice be Course for when I damage a blade, followed by Extra Fine as my third pick.
Other option is to pick up one of those diamond sharpening rods that are the size of a pen and retract out.