They are sometimes quoted at equal size, but generally Unicron is about half the size of Cybertron in there respective planet moes, so it's generally accepted that Unicron is the smaller of the two as far as I know.
Re: Does anyone know the sizes of the Transformers?
I've spent quite a bit of time looking into this. Please note my answers are from the G1 UK comics, and partly from well-drawn parts of the cartoons (esp Transformers the Movie).
I got my answers from looking up answers in the letters page, looking at the Transformers Universe, reading the comics (once or twice heights are mentioned), the Annuals, and scaling off the TF Movie. Overall I spent a full week on this.....no insults please.
OK, here we go:
Small Transformers (Bumblebee, Rumble etc) seem to be around 8-9 feet tall. I'm pretty sure they'd beat the Terminator in a fight due to better technology etc, and they're a bit bigger as well.
Medium sized Transformers (Jazz, Ironhide etc) seem to be around the 20 foot mark.
Medium / Large robots (Starscream) seem to be about 30 feet tall.
Large Robots: (Optimus / Megatron / Shockwave / Soundwave / Dinobots in robot mode) seem to be about 35 feet tall. In the TF Universe Shockwave is described as maintaining equal size with his gun mode which is "over 35 feet long". So 35 feet seems right, especially when I compared with Devastator who's size is mentioned.
Giant Robots vary a lot:
Bruticus is one of the shortest, 50-55 feet.
Devastator is one of the tallest at "over 60 feet tall" (TF Universe) which by my measurement = 64 feet. Menasor/Defensor/Superion are probably in the high 50's.
Predaking is the largest (and strongest) of all the combining Transformers and his height is specifically mentioned in the comics as 80 feet.
By my measurement Omega Supreme is 95 feet tall. Contrary to the cartoon's depiction, Trypticon is about the same size, though shorter because of his design.
Metroplex's height is specifically mentioned at 697 feet.
Unicron is as big as a small planet, and he grows as he eats.