One of the greatest things about anime it that it doesn't have to follow the rules of reality. Whether it's four-foot tall girls wielding swords the size of a 747, or shy nerds somehow getting a harem of girls obsessed with them, anime doesn't play by the rules, man. Of course, this means that there are a lot of weapons in anime that are so huge, they would actually be a massive liability for anybody to wield in real life.
A minor quibble: giant robots don't count as weapons. What does count are the weapons wielded by giant robots which are, some f*cking way, still way too big.
Here are the most absurdly, improbably, and ridiculously big weapons in all of anime. This is going to be sweet.
Dragonslayer From Berserk
Guts is the great-granddaddy of big dudes wielding bigger swords, and his chosen weapon looks the part. Huge, simple, and dangerous this thing looks like it is ready to mess up somebody's day. BIG time. Get it?
Punisher Cross From Trigun
Nicholas D. Wolfwood’s Punisher Cross is the Swiss Army knife of guns. You know, if instead of being portable, Swiss Army knives were the size of an adult person. In addition to being massive enough to use as a shield, the Punisher also contains a machine gun, a grenade launcher, and small armory of hand-held pistols.
Harkonen From Hellsing
Harkonen is a massive anti-tank rifle that Seras Victory wields like a regular rifle. As if that rwasn’t rad enough, she dual wields them later in the series. Best of all, this crazy bullsh*t happens in a setting where she is one of the weaker vampires in the setting. Guess size doesn't matter in the Hellsing universe.