...Well, first off, [Daitarn III] was just shy of four hundred feet tall, making him one of the largest super robots of the era and still one of the largest of all time. Granted, the [Daitarn III series enemies] were exactly the same size most of the time, but still-Daitarn is significantly bigger than the king of the monsters Godzilla, the universe-slayer Ideon, and nearly every other kaiju and super robot you can think of-the only exceptions to this I can think of are Danguard Ace (650 feet tall), Gunbuster (~812 feet tall), the titanic SDF-1 Macross (I'm unclear on its actual height, but in battleship mode it's about 3/4 of a mile), and the really ridiculously large things like Dark Nova/Star Giant, Primus, and Unicron (all the size of small planets)...
I should have known better than to overlook Tengen Toppa Gurren Lagann.
Why? Because the series pretty much happily defied physics and logic with more ease and willfulness than Western comic books or even most of the crazier anime, and they eventually took the progressively increasing size of their mecha to the point of having one the size of a planet... and then took it one more step, to Tengen Toppa Gurren Lagann, which was a combination of all previous mecha used by the protagonists. Yes, a mecha with the same name as the series. How big was it?
Behold. (WARNING: Lots of scantily clad females. This is part and parcel of any Gurren Lagann sequence. Further, sorry for the slightly hentai-esque bit with the creepy negative-space guy.)
Not symbolic. Not an exaggeration. (I feel like a Silver Age comic book cover.) According to Wikipedia, the official sourcebooks explicitly state that Tengen Toppa Gurren Lagann is 10 million lightyears tall. That is approximately 100 times the diameter of the Milky Way Galaxy, and 10,000 times its thickness. Imagine the collateral damage. (The battle was supposed to take place in some kind of alternate universe, by the way, which is why civilization and life in the universe didn't instantaneously end as a result of the mecha's combination sequence. And why there was a ground.) The little things that the big shadowy enemy are chucking at it are entire galaxies. The enemy was attacking because they were worried that the fictional energy that powered the series mecha, which explicitly defied the laws of physics (particularly conservation of mass and energy), might destroy the universe. Gee, what are the chances?
Ironically, the Ideon (previously mentioned) could still destroy Tengen Toppa Gurren Lagann.