knowing how the kamis work, it would of course be 18" long (minus the shaft) and weigh 2 lb.
i'd like a nice leaf shaped one about 8-12" blade, central stiffening ridge both sides and a 1.25" dia. socket for the shaft.
samples:
here's one i've had for years (not HI tho

)
socket is only 3/4" dia, could be a bit larger dia socket. 12" blade, section near 3" socket is 1/2" sq.
central ridge distal tapers to point, blade 1 5/8" max width.
could be fancier:
here's a fullered and pierced bronze age spearhead. (repro)
as cast, 8" LOA. needs a bit of reaming out of the socket, and edge grinding to reduce the weight. this one does weigh 2 lb.
could also be much simpler: one from a tribe in eastern india.
flat steel blade, tail piece.
which brings up the next comment: a spear should be accompanied with a corresponding tail piece to balance the head and provide a convenient bit to stick in the ground when needed.