Ok, Combiners dont count. But how big can a signle transformer be ?? So can there actually exist a Transformer who transforms into a Aircraft Carrier ?? As robot he would be more than 1000 feet high, thats just impossible. Thats also impossible to imagine that such a giant lived on Cybertron. So how big can they be ?? I think that the maxium is maybe 150 feet.
Giant robots from space seem kind of like an impossibility, so I dunno what the limit would be. In fact, I don't think there is a limit other than whether it fits in the story.
Remember that there have been transformers the size of bases, aircraft carriers, cities, and entire planets before. I wouldn't say it's impossible.
I think that giant carriers and spaceships on Cybertron are just machines and no robots. But the biggest current movie transformer is Depth Charge, who is 150 feet high.
Since there is no mass shifting in the movieverse (the Allspark could but not the TF's) a size limit would make sense. The biggest we've seen onscreen, not counting Devastator, was Demolisher. If you count "expanded movieverse" I'm not sure.
And before ROTF, we didn't know that movie TFs could share identical forms, nor did we know they could combine, nor did we know they had supernatural telekinesis powers, spacebridge capabilities, etc. Basically, just because we haven't seen it yet doesn't mean it can't exist in the mythos yet. Since its fiction and not really bound by the laws of physics or whatever, the sky's the limit. Hell, maybe we'll find out some of the carriers and spaceships on Cybertron ARE robots after all. Depthcharge could be just the beginning.
I don't believe there is a set limit to how large a TF can be. We've already seen massive bots in TF history like Unicron, Primus, Metroplex, Fortress Maximus and Trypticon. All of which have been either the size of a planet or the size of a Transformers city.
Primus is basically the Good version of Unicron. He's the god of the Transformers, hence them saying things like "For Primus' sake". In some continuities, he is the living spirit of Cybertron, and once awakened, can transform the entire planet just like Unicron. He got a cool toy and everything.
Difficult to speculate right now since we are now in Bay-verse, and there may be more to know than imagined. Secondly,Cybertron has got to be a number of times larger than Earth, so giant robots on Cybertron is far from "impossible."
Considering the enormous lifeforms that have existed on our own planet, Transformers hundreds of times larger than the usual 25 feet tall robot don't seem unusual.