While I dearly love my Harpy and for some of the box tasks; it is perfect (pull-cutting the side tape without slicing into the contents), I have to go with the Calypso Jr in any of its incarnations(my fav is the micarta). The handle size gives good grip and the flatground blade cannot be beat for long runs thru designed-tuff material. The blade is both pointed and wide, giving good starting penetration and then easily maintained courses butt not tooo wide so that you can follow flowing cut lines. The length gives enough surface to saw with if necessary butt doesn't over-penetrate to puncture delicate contents.
I do tend to wax on about the late great Calypso Jr. I don't see another offering by Spyderco that fills this niche.
Oh yeah.... The Calypso is way over 3.5 inches. (butt it has its great points too)