I'm leaning toward Metroplex being bigger. (from a certain point of view)
-G1 S3 "The Ultimate Weapon". When Blades dropped First Aid onto Metroplex to fix the cog. There's a shot where First Aid (struggling to hang on) is beside Metroplex's head. It it appears to be 4-5 times bigger than First Aid.
-Assuming First Aid is as tall as Cerebros, and Spike (or any other human) fits in as Cerebros' head, I'll assume Spike can fit into First Aid's head (if First Aid ever wanted to be a Headmaster).
If First Aid were to transform into a head, and even including the size shift where he becomes the size of an actual ambulance (twice the size), First Aid's "head mode" would still not be as big as Metroplex's head.
Thus I can conclude that Cerebros in "head mode" is smaller than Metroplex's head.
If heads are proportional to the bodies they're on, Fort Max's body would be smaller than Metroplex's.
Thus Metroplex is bigger than Fort Max.
Could you imagine the real Metroplex toy if Metroplex's head was 3 times bigger than the current Cerebros? Since Fort Max is already 2 feet tall...