There’s a lot of big swords in video games (many of them direct copies) but I’d submit the sword of Gustave the Steel in SaGa Frontier 2:
In the setting of this game magic is done by channeling through organic matter, so all the weapons and armor are commonly made out of the likes of wood; Gustave is bastard nobility, disowned because he’s born unable to do any magic at all. As a kid he takes up with a blacksmith, a profession which here makes utility tools like kitchen knives you wouldn’t bother with magic for, and after apprenticing for years convinces his master to let him try forging a steel sword, a bizarre novelty.
This one’s only practice. The real sword I intend to forge will be three times longer.
Can there really be a sword as monstrous as that in this world?
Yes. I can see that sword. . . somewhere out there.
SF2 is a story covering decades of time and generations of characters, and Gustave’s part in it is his rise from exile to reclaim his birthright; as you follow him, and he gains in political and military power, the Gustave Sword keeps getting bigger and bigger as he reforges and adds onto it off-screen, until by his sole late-game appearance as the middle-aged world-famous Lord Gustave XIII it’s a huge slab of metal bigger than any character’s sprite.