Luckily, a weapon's size and the requirements to be able to use it are separate tags.
The easiest way to do this would probably be to create a copy of the two-handed sword and just increase most of its values.
Increase the weapon's size by a considerable amount, increase the minimum required size to wield, and increase the contact area, penetration, and velocity modifiers.
For reference, here is the two-handed sword's raws, with some notes on the tags:
[ITEM_WEAPON:ITEM_WEAPON_SWORD_2H]
[NAME:two-handed sword:two-handed swords]
[SIZE:900] < The total volume of the weapon.
[SKILL:SWORD]
[TWO_HANDED:77500] < Anything below this number in size will have to use the weapon with two hands.
[MINIMUM_SIZE:62500] < The minimum creature size necessary to use. A standard dwarf is size 60,000 and a human is size 70,000.
[MATERIAL_SIZE:5] < The amount of materials used to create the item.
[ATTACK:EDGE:100000:8000:slash:slashes:NO_SUB:1250] < The first number is the contact area, followed by penetration. The last number is the velocity percentage, multiplied by 10. So 1000 is 100% velocity.
[ATTACK:EDGE:50:4000:stab:stabs:NO_SUB:1000]
[ATTACK:BLUNT:100000:8000:slap:slaps:flat:1250]
[ATTACK:BLUNT:100:1000:strike:strikes:pommel:1000]