Just reading this, and it says that Maximus is the second largest Autobot... Who is bigger than him?  Maybe Omega Supreme?  I dont know if a shuttle and launch base could be as big as a 'city'...

Metroplex is probably bigger.

There is never any set-in-stone 'size' of a Transformer that runs through all continuities. The scale of a transformer is rarely the same between artists and even the same artists draw a single transformers bigger or smaller from issue to issue. *coughs* Don Perlin *coughs*
 
Metroplex is certainly the biggest single Autobot by a long long way and by my reckoning Omega Supreme pips Fortress Maximus.