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.
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).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).
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.