"10 Facts About the Imperial Fists"

[Lord_Caerolion]

It is debatable when we've had multiple sources claiming different things to be "this is the biggest".

[Red_Shift]

The Phalanx is the largest loyalist warp capable structure in the Imperium. Simples.

[Brother Tyler]

While the Rock, which is probably larger than the Phalanx and which is also capable of warp travel, isn't a "structure" (i.e., a man-made construct) so much as an accident where a structure smaller than the Phalanx happened to survive when a huge chunk of the planet on which it was located remained stable in the aftermath of the planet's destruction.

 

So we're really arguing semantics and both positions are correct when phrased appropriately.

 

Simples.