A lot depends upon the type of music and - especially - on the harp player. You really need to talk to him/her first. If you're talking blues harp, I'd be surprised if the player didn't bring his own mic with him, probably in some form of bullet mic or other gritty-sounding dynamic that they'll cup in their hand right up to the harp himself, no stand desired or required. Also, with such
a blues harp setup, typically you want to run that mic into some kind of small, gritty tube amp and then mic that amp with something simple like an SM57.
If they don't have their own mic with them, just give them a dynamic of a style that they'll be comfortable holding. Some prefer a ball mic like a 58, some prefer something not quite so fat, most probably won't care much because they can hold on to just about anything.
If it's chromatic/folk-style or country melodic harmonica, then they might or might not go the hand-held route and prefer to play to a stand. If so, just about anything that won't make the high end brittle will work, but you want to set the stand and mic to fit two conditions: one where the player is not necessarily blowing directly into it but rather just above or below it, and can do so at a head angle that's comfortable for that player (some like holding their chin up, other like burying their chin in their chest, many are somewhere in the middle.)
G.