You can't wield it at all
According to the rules on weapons:
Inappropriately Sized Weapons: A creature can’t make optimum use of a weapon that isn’t properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. [...]
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.
The large lance in the hands of a medium creature is changed to one size above two-handed, and therefore can't be wielded at all.
The lance has the specific ability:
A lance deals double damage when used from the back of a charging mount. While mounted, you can wield a lance with one hand.
There are two possible interpretations of this:
- Specific overrides general, and you literally can wield any outsize lance in one hand. However, this leads to the absurd result that you can a Colossal lance in one hand, which is clearly not rules as intended.
- The rule is clearly intended to refer to a lance of the bearer's size. It doesn't say "the lance decreases by one size class", you simply gain the ability to wield it in one hand on the assumption that you could already wield it normally. There's no rule in the lance description which says "...and you can wield a Large lance in two hands", only that you wield a normal two-handed lance in one hand.
Given these options, the rules intent seems clear that a medium creature can't wield a Large lance at all, even while mounted.