Personally, I would recommend a 5" to 6" blade, that's a little bit thick with a beefy point, and a Spec-Ops sheath. More than that isn't really necessary, and may in fact be too big to easily carry when you factor in all the OTHER gear you have to carry as well. Back that up with a Multitool in the pouch on the sheath, and you'll be good to go.
There are a few Busses out there that would work well, and Swamp Rat and Scrapyard blades also.
If it was me, I would go with a Busse BATAC, or Swamp Rat Ratmandu, or Swamp Rat M6, or possibly a Busse SJTAC (largest I would consider for this use), or Scrapyard Scrapper 5. Any of these is big enough to use on a man if you have to, and small enough that you can use it for other things, and it won't be a total pain to carry.
Here is the M6 I sent to Afghanistan with a long-time friend of mine (although he grabbed my ACU sheath instead of the tan sheath... identical except for color, and the ACU sheath still had the tie-down line):
When he told me he needed a good knife, I bought a couple new M6s, and brought several knives to him of varying sizes, so he could pretty much choose what works for him. That's what he chose. I brought several over because, after 14 years in the ARMY I figured he would probably have a good idea of what works for him.