Even thrusting swords were used as "lances" in RL, so there's really no minimum length, except that when you're using a longer weapon you get to strike first. The idea of a "lance" is that if you strike with one in the same direction your mount is moving, the blade will move faster than it would if you were charging on foot. RQG isn't a physics simulator, so only proper lances work in it for the purpose. As for the "pikes", bayoneted muskets were used for the purpose, so a short spear should work too. The required length needs to be long enough to reach the target obliquely from the ground, so the under 3' long Zulu iklwas wouldn't be useful. In RQ3 all spears worked for cavalry and infantry. I'd call the long spear the Goldilocks weapon, eg. the Greek hoplite's Dory was 2-3m, as were the 19th and 20th centuries' cavalry lances.
My group is now playing RAW RQG, but after the first scenario is done, I'll probably pick some houserules from my FrankenQuest, one of which was that charging lancers (or people using other suitable weapons) don't use the mount's damage bonus, but instead their own, and the weapon's basic damage is rolled twice. Iirc, Call of Cthulhu lances used the same rule.