from the wiki on Unicron:
Unicron's size is never specified in any canon materials and has been left to fan speculation. "Planet size" is often stated but that covers so large a range as to be useless for anything exact. In Transformers: The Movie, Unicron appears to be anywhere from several kilometers (such as when handling Galvatron) to several hundred kilometers (when attacking and destroying Lithone and Cybertron's moons, and attacking Cybertron). A planet-mode diameter of 1000 km has been suggested [3] and robot-mode dimensions would follow from that estimate provided that there is no change in size (compression or expansion) common among so many transformers. In fact, one scene in the movie shows Jazz driving out of one of Unicron's eyes, and by comparison, Jazz is just small enough to fit through. Unicron is also small enough to stand on Cybertron's surface(some fans have also noted that, in comparison to scenes from the movie, the 3" tall Micro Machines Titanium series Unicron figurine is to correct scale with the Cybertron planet mode of the Transformers: Cybertron toy Cybertron Primus). In season 3, it is shown that the eyes from Metroplex and Trypticon (both city-sized Transformers) are the perfect size to use as replacement parts when Unicron orders the ghost of Starscream to steal them. Oddly later in the episode when Trypticon moves Unicron's head, the head is larger than Trypticon's whole body. In the storyline of The Ultimate Doom, Cybertron appeared to be smaller than Earth's own moon (and by extension, this would indicate Lithone would be incredibly small for a planet), making Unicron's dimensions in the cartoon harder to define.
Interestingly the comic version of Unicron may be far larger. In the first issue of Marvel Comics Generation One Transformers comic, Cybertron is said to be around the size of Saturn (which has a diameter of roughly 120,000 km at the equator). As Unicron's planet mode is seen to be as large as Cybertron (if not larger) in the comics, this would suggest a diameter of 120,000+ km. The Dreamwave equivalent of Unicron may be the same size, as the Cybertron of the Dreamwave universe was also said to be the size of Saturn. If Unicron is the larger of the two, as an overwhelming odd character, he could be the size of Jupiter.
In the Transformers: Cybertron comics, the tank version of Unicron is only the same size as an average Transformer - actually being shorter than the Autobots he fought, like Sentinel Maximus and Optimus Prime.
So, depending on the version, this guy could be slightly shorter than Optimus, or Bigger than Jupiter. 😑
