OK, so if a cubit is about 18 inches, then a span (half-cubit) is 9 inches, and a palm (a third of a span) is 3 inches.
This actually makes Polybius's description the same as Livy's description. The same length for the wood (a span) and the iron head is 2 palms, not 2 cubits. I guess the Wikipedia article needs fixing: they quoted the wrong translator.
And if you go all the way back to the beginning of this topic, Hondero quoted somebody that translated it right. The dart is about 15 inches overall, and its length is about
2/
3 wood and
1/
3 iron. This matches
Bertrand's sketch
(for which DuckOfDeath provided a citation: Alexandre Bertrand, "Le Kestre ou Kestrosphendone,"
Revue Archeologique, Vol. 27, 1874, pp. 73-78 and pl. II). Hondero's reconstruction looks closer to half and half.
Whew. Glad we finally got those measures straightened out. Though Hondero had it right, from the start...