Please note that these are my interpretations of the sizes and that they will thus vary with other, official media. Those typically just do 'whatever' where sizes are concerned anyway, which means there IS no real size convention for any of the media apart from what's an average size. Those tend to be 'default to nearest average'-sizes though and are as such still random.
As far as I'm concerned these are the actual sizes based on logic, concerning character types I might use in my fancomic. Though I might adapt the ones here for logic sake as well (not for convenience) and would then run consistently with that. One example would be Micromasters with large altmodes: those might actually end up being slightly larger than Minibots to allow humans inside (even if cramped, the altmodes need a function).
CITYBOTS Personally I don't think all Citybots are equal in size. Haven't even included them all in the chart (yet) since it'd get a bit cramped in the image on one hand and because I'm not fully determined to which size convention I'll use for them. For now, XII is a placeholder based on the G1 cartoon.
However, I'll share with you my logic on them and what they'll probably be:
Scorponok for example seems to be more along the lines of a Combiner to Guardian Robot due to his humanoid Headmaster.
However, Fortress Maximus would be a bit larger again (due to cerebros having a robot headmaster for head) and would probably end up on the Omega Supreme scale.
Alternatively, I could run with the Japanese variant of Headmasters and ignore the Nebulons and Rebirth (!), which means they'd be robots not organics; meaning I could scale Zarak to any size I'd want allowing me to use Scorponok on Fort Max size. If both have robots for heads I could make them even larger, but then the headmasters would have to be the size of Optimus or larger, which isn't my intention.
That leaves a few more citybots.
Sizing for Metroplex though would be independent of Metrotitan, as I'd base Metrotitan and Grandus around Micromasters. Metroplex and Trypticon could be based on regular size TFs, which would make them even bigger (note that this sizing would also affect some, but not all of their partners!). So yes, that'd mean Scorponok and Fort Max would not be the same size as Trypticon and Metroplex as each would be scaled on a different type of user. From this the following size comparison follows:
Als note that Grandus will have no direct sizing relation to Star Convoy, even though the toy allows them to link up in 'vehicle mode'. IMO though a truck pulled aircraft carrier of humongous size is silly and wouldn't work. Grandus works better as a semi-subsurface base of operations.
When I conceived of what the actual size of citybots like Metroplex and Fortress Maximus would be, I imagined transformers as big as mountains. Wouldn't they have to be that large to comfortably house dozens and dozens of transformers 20ft tall or more? What's the point of even calling them citybots if they're not at least the size of a large base for transformers? Sadly, transformers has always been plagued by scale issues. They seem to 'magically' add size and mass in some of their modes. But, honestly, a citybot would need to be thousands and thousands of feet tall - miles high. The only piece of art that is within a shadow of that concept is this picture circa 1987 - found on the back of the transformers bought in that era (think Headmasters, Monsterbots, Targetmasters, etc.) At least in this picture you get to see how citybots would have to fight - in space. They are so enormous if they were constrained by Earth's gravity they'd be incredibly slow (and destructive): botchthecrab.com/gallery/box_b…
I know this is old, but this popped up on a search. I don't know if you've updated this sense, but Optimus Prime from Generation One is offically 6m or about 20ft tall. Also, cassettes were always shown to be about human sized. Most of the G1 autobots came in around 12-15ft based on the size of Prime, Bumblebee would have been about 7-9ft based on his size compared to humans and the others.
I only posted this because I saw other people had asked about sizes in feet and trying to figure out where they would fit on your chart.
Well if we'd follow the cartoon, Primus is pretty much moon size in planet mode. That would actually make him bigger than Unicron, who in is Cybertron size in robot mode, which tends to be three, four times bigger. Unless of course, most of Cybertron would be a shell. >.>
Consider though that the spaceship in the movie that went through Unicron's eye was pretty big in relation to Transformers, but only as tall as the height of the Unicron eye... So... you could try to make some sort of extrapolation from that, but it'd be really tricky.
Comics... Don't even go there, but UK G1 had some ideas about Unicron's size in relation to the Junkions (who would be "Normal G1 size")...