Fortress Maximus, in the show, was about as random in size as any other tf. he was also of a shifting relative size. however, i believe he is portrayed as smaller than metro in the stories.
i can give you a complete comic rundown
Fortress Maximus was leader of the Autobots on cybertron after the departure of O.P. he eventually gathered a group of peace-seeking autobots and departed to the peaceful planet nebulos. at this point, FM was the size of a normal robot. he was an architect until the war. his weapon base mode was a hovercraft thing. you'll notice the toy has wheels in this mode. okay. so they go to nebulos, but guess who follows. yep, scorponok and the 'cons. war breaks out, the nebulans get all PO'ed. fort max and some other bot's give their heads to teh nebulans as a sign of good intent. nebulans bioengineer themselves as headmasters to fight the cons. cons team up with some corrupt nebbies and do the same. they eventually beat the con's off nebulos and leave. fort max, terribly injured, is reconstructed on the way to earth into huge, city sized FM with the itermediate robot cerebros. they get to earth, some unimportant stuff happens, FM's binary nebbie dies. spike witwicky eventually takes the binary helmet and plays hero boy for a while. meanwhile, other stuff happens and spike puts FM in storage and leaves the helm. comicbooks later, galvatron lands on earth and starts beating stuff up in canada (the last autobot). FM calls to spike, who eventually puts on the helm again and becomes one with the only autobot left alive on earth. interesting battle with galvatron. end G1. G2 i don't remember so sharply, but that's when max dies.
he is really quite a deep character, i could not portray that in my summary, as i had to cover a couple years worth of comics. if you'd like to know more about the toy or story, let me know.