When a weapon is inappropriately sized, it changes the amount of effort required to wield it.
The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder's size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon's designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can't wield the weapon at all.
Lances are two-handed weapons, and have a specific rule that overrides the general rule:
While mounted, you can wield a lance with one hand.
This exceptional rule does not change the classification of what the lance actually is; for a lance appropriately sized for a medium creature, it would still be a two-handed weapon but it would be wielded in one hand. In other cases, it would follow that the character can still wield the lance in one hand, even if not in two.
With other similar abilities, the text usually qualifies that the weapon must be appropriately sized for you. I assume that should be inputted here? Are there rules or an FAQ somewhere my search didn't catch?