Weapons that are more than twice as long as the character's height can't be used in combat at all, except bracing spears for a charge. Melee weapons that are more than three times the character's height are completely unusable by a single character, as are weapons longer than the character's raw, undivided Strength rating. Magical assistance may remove this limitation; so could two characters working together to brace an extremely long spear by combining their Strength scores.
Edit: Per a comment by Prince Herb, I've tweaked this and condensed weapon lengths to:
- 1-hand weapon <= Strength/3 OR character's height
- 2-hand weapon up to 4 x height OR 2 x raw Strength
- two-person pole arm > 4 x height OR 2 x raw Strength