Anyone want to take a guess as to a mutalisk's size?

[Eligor]

We could always assume that the description in the manual is data collected by Terran scientists before they encountered the exceptionally large Scourge specimens. In any case, there's really nothing to prevent the Swarm from evolving larger Scourge at some point (a bit of an aside, but I'm pretty sure that there generally are many more unique Zerg strains than what we see in the game).

[Visions of Khas]

I just noticed the Brood Lord looks like a giant scourge.

[Josue]

Which I think pretty much means we can throw out any Blizzard cinematics as far as scale reference goes...

Seriously, I think we can safely state that we know BCs are huge, able to carry troops and dropships and probably squadrons of Wraiths. It's also safe to say that Scourge are smaller than Mutalisks. If you take these things in combination with the various screenshots and cinematics...it is just inconsistent.

IIRC, the manual stated that scourge could cripple enemy fighter squadrons. This would imply that they are comparable to wraiths in size, but doesn't necessarily preclude larger varieties of scourge, like the one seen in the cinematic.

We could always assume that the description in the manual is data collected by Terran scientists before they encountered the exceptionally large Scourge specimens. In any case, there's really nothing to prevent the Swarm from evolving larger Scourge at some point (a bit of an aside, but I'm pretty sure that there generally are many more unique Zerg strains than what we see in the game).

I just noticed the Brood Lord looks like a giant scourge.

All this just returns us the main question:
How big is big?
I think we must ask Blizzard for official information, we are just making conjectures without reaching the real truth behind the wide variety of "evidence" all the cinematics, pictures, etc represent.