Back in 2000, Hobby Japan (known for it's very Gundam biased ways) gave it's readers a scale chart to show which are the largests robots and mechas of all time. The results were quite interesting.
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Shown above is Unicron (behind him is Jupiter, as in the planet for scale), then they move onto smaller mecha in a whole other size class (Compared to the Golden Gate Bridge of San Francisco). From left to right, the SDF-1 Macross (Of Macross/Robotech), Metroplex (Of Transformers 2010...... odd they left out Fortress Maximus) and below them all is Mazinger Z (of you guessed it, Mazinger Z). Keep in mind this ain't meant as a countdown from biggest to smallest, just to give an idea of different sizes (otherwise we would have Gunbuster somewhere).
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They then go onto a smaller scale yet again (comparing to the Twin Towers pre 9-11, this again was done in 2000). From left to right we have Danguard (of Wakusei Robo Danguard Ace), Ideon (of Space Runaway Ideon), Combattler V (of Combattler V) , the RX-7 Gundam (of Mobile Suit Gundam 0079) and above is Yamato (of Space Battleship Yamato/Star Blazers).
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So it looks like our beloved Unicron is the king of the robot size class. Also I don't want to hear about Tengen Toppa Gurren Lagann being bigger because the final form was not a robot but the combined form of Spiral Energy (like say the combined Minicon sparks in Transformers Armada that made the "Green" Unicron), I love Gurren as much as the next guy but it kinda ends up in a different grey area.. Either way, it's good to be on top.