Just a random thought but if we take away the mass shifting and the transformer in question just changed shape not size, who do you think would be the biggest robot. I'm taking titans out of this so just standard Cybertronian's. I would've normally said broadside as he's an aircraft carrier but his jet mode would knock that down, but as I've taken away mass shifting he's either a really big jet or a really small ship so he's disqualified also.

 

G1 Skyfire probably since he was ferrying around Autobots like they were almost human-sized. The Masterpiece even includes little miniature figures of some Autobots that are in-scale.

 

Astrotrain as he ends up big enough to have Devastator fighting with the others, inside of him.
 

It really depends on what scale you are going with.. "reality" alt. modes or what we've seen in the media.

 

If we're judging purely by robot scale without going into Combiner/Titan and above characters then there's only one other bot I can think of.

Grand Ascension Megatron, calculated at 26.7 Meters (87'8).

 

I was thinking the real adaption of their alt modes so if a car and jet transformed the jet robot would obviously be huge to the car robot but then you add silverbolt for example he was a Concord jet so he in turn would tower over starscream who would tower over jazz on so on

 

Without touching the Titans, there's a limit to what would be the largest, and most certainly, my guess is going with a plane.

The Concorde hits the line of where things would stand, as it would dwarf any shuttles, along with those along the same types, Armada Jetfire maybe?

Boats would be next most probably.

If you go with live movies, the constructicons and such were massive, but a couldn't say which was largest without looking it up.

 

Agreed... At the toy size, he's absolutely out of scale with everything, but on his own, he'd be Titans class at the very least.

 

Your mum.

Oh wait--

 

This is intresting

 

tidal wave has to be in there somewhere even though his height and mass fluctuate to being slightly taller than Jetfire in the armada cartoon

 

Silverbolt being a 202 foot long Concorde would naturally be one of the biggest fliers and Cybertronians in general. Even if he lost some of the length in root mode due to pieces overlapping for his legs, arms, torso, etc, he'd outsize all other normal (non titan, combiner, etc) Cybertronians.

 

I'm making the following assumptions: I'm excluding citybots and planets, but other big guys like Omega Supreme or Sky Lynx are ok. You may of course ignore them if they don't fit the criteria. Size is determined by alt mode without mass shifting, alt mode is based on real world vehicles, so Astrotrain is as big as a Nasa shuttle and not able to fit all the Decepticons including Devastator. Mostly G1 since I'm most familiar with that continuity and the comparison makes the most sense with Earth alt modes.
Anyway, obviously Broadside would win but he can't exist in a world without mass shifting.
Both Silverbolt and Octane transform into planes similar in size, around 50-60 meters in lenght and 180 tons in weight. Sky Lynx becomes a shuttle, the orbiter alone would be smaller but it's only half of the robot so he might be a similar size. Skyfire is a fictional plane so it's difficult to judge, if we assume he can carry autobots he will be really big, if he's supposed to have a human sized cockpit his size would be much smaller. Then we have Omega Supreme, a fictional rocket, if we take as comparison the Saturn V it's 110 meters tall and weights 3000 tons, plus we have to add the tank and the launchpad, so huge.