Apologies if I'm resurrecting a 'dead' thread...
Suited Marines have to be at least two metres tall. An 'average' human (today) is 1.8 metres tall. A marine suit would need to be able to accomodate a range of heights. And this is assuming that better medical treatment hasn't caused humans to grow even taller (but we'll let that slide.) A three metre tall Protoss Zealot is really quite tall in comparison. (And has anybody seen how thin the suit helmets are? Low velocity shrapnel protection only, hmm?)
(Interestingly enough, you could probably make a good estimate of the height of a Missile Turret since, according to the status diagram in the game, there's a guy inside. And thanks to Ghost, we can now tell how big an actual Bunker is, although with firing 'slits' that big, the Bunker is of dubious value.)
Goliaths (being walkers and all) are probably around five metres high. The cockpit is probably 2 metres high (needing to house a reclined human being plus equipment). The legs can easily extend the height by another three metres. (On a side note, the Goliath is a flawed fighting machine. Too tall to hide, too slow to get away, and the legs are far too vulnerable. A smaller wheeled or tracked vehicle would have done better, but I guess it would not have been as kewl.)
The Arclite Siege Tanks are probably about the same height as the Goliaths.
Obviously, Dropships then must be somewhere like ten metres high (tail fins and all). The Wraiths are probably a bit shorter than ten metres.
Saying that, I think Behemoths can be no less than 600 metres in length, if the flight bays in the hammerhead are ten metres tall (enough to handle Wraiths but not Dropships). This makes the hammerhead some 100 metres tall. From there, my estimation of the length gets a bit fuzzy. Note that if the flight bays are able to handle Dropships, then the bays must get rather taller (perhaps 15 metres), increasing the length of the Behemoth further (say around a 1000 metres.) Personally, I go for the 1000 metre figure.
The E3 2005 cinematic for Ghost has a rather nice scene of an Ultralisk surrounded by Marines. (Makes me laugh. "I shoot my puny not-so-Guass-rifle at you!" and "Let's stand suicidally close to the Ultralisk while we're shooting at it!") From that, I'd say the Ultralisk is maybe eight to ten metres tall. Time to bust out the RPG's...
From that same cinematic, Zerglings are like 1 metre tall (not including those two claws sticking up over their heads.)
Hydralisks seem to be 5 metres tall from that Ghost screenshot. SCV's are probably Hydralisk-sized.
I wonder how big Valkyries are...
