Doing some quick samples, it looks like the weapon adjustments based on length follow roughly the same pattern. If you subtract 3 from the length in feet for several weapons, you get the numbers on the "Ways of the Earth" table. If you add a couple simple rules, it's closer:
- axes and flail/chain weapons get +2 vs. shields
- thrusting and missile weapons get -2 vs. shields
Those are cumulative modifiers, unlike the ones in the linked table.