The ability to understand and use any and all giant weapons with the proficiency of a master. Sub-power of Weapon Proficiency.
Giant Weapon Intuition/Mastery
The users are masters in the use of weapons that are notably, sometimes massively so, larger than the normal size for the users size. This gives the user better striking power, wider range of attack and destructive force.
As a Heavy Blade class, BlackRose (.hack) wields a massive double-handed sword with ease.
Takumi Atsuta (Battle Moon Wars) wields the high-tech Futsuno. With it, he is able to deliver strikes powerful enough to, as he claims, even "cut gods". He can also activate the weapon's Ignition Release mode to increase its power.
Cloud Strife (Compilation of Final Fantasy VII) with his trademark Buster Sword.
War (Darksiders) always carries his immensely powerful Chaos Eater in the never-ending war between the kingdoms of Heaven and Hell.
Throught his life, Kratos (God of War series) wielded a wide variety of weapons, notably large and powerful ones such as the Blade of Olympus.
Magnus (Kid Icarus: Uprising) easily wields his massive sword, the Magnus Club, with ease.
Whenever Kirby (Kirby series) inhales a Super Blade Knight, he gains the ability of the Ultra Sword.
Whenever Kirby (Kirby series) inhales a Super Bonkers, he gains the ability of the Grand Hammer.
Impa (Legend of Zelda: Hyrule Warriors) is able to wield the Giant's Knife with speed and strength great enough to dispatch an entire hoard of enemies and powerful opponents.
Solid Snake (Metal Gear) easily wields Crying Wolf's massive railgun with his bare hands.
Fortune (Metal Gear Solid 2: Sons of Liberty) can wield a large railgun with ease.
...can use Rollers, which are far larger than your typical paint roller or paint brush...
...and the Inkzooka, which is far larger than your typical bazooka, with proficiency.
Agent 3 (Splatoon) can use the Inkzooka and one Roller weapon...
...the Hero Roller, with proficiency.
Mecha-Frieza (Super Dragon Ball Z) uses a massive rocket launcher mounted on his shoulder to deadly effect.