You can't use it, it's too big.
Each increase in size category increases the amount of effort required for the weapon. A Medium warhammer is a one-handed weapon for you, a Large warhammer is two-handed, and a Huge warhammer would require more than two hands. Unless you have a special feat, or more than two arms, you cannot wield that hammer at all.
In any case, upsizing a weapon does not ever grant reach. It's not designed to be wielded like a spear, especially by a creature your size, so you are not able to take full advantage of its increased length. (Similarly, if you wield a larger-sized reach weapon, it doesn't give you any more reach than a regular reach weapon.)
[Edited to add]
Ah, I need to type faster, the mention of enlarge snuck in ahead of me.
If you are changed into a Large creature, you can wield the warhammer as a two-handed weapon with a -2 penalty to hit. It still does not grant you reach.
Note that you'll have to put down the hammer before you're targeted with the enlarge spell. If you're carrying it at casting time, it will increase in size along with the rest of your gear, so you'll become a Large creature with a Gargantuan warhammer which you still can't use.