How tall is Horus Lupercal in Warhammer 40k?

Answer 1

If a 'normal' human is considered to be roughly 6 ft tall, and a space marine is at least half as tall again (9 to 10 ft), then Horus is very roughly, at a good head taller than his captains, about 11 or 12 ft tall.

However... It has been shown throughout the literature that the 'stuff' primarchs are made from is not like humans or space marines at all and therefore is not fixed. Some primarchs are said to be 'tall' (Mortarion and Sanguinius), some are hulking (Leman Russ and Angron) and at least one (Alpharius Omegon) is indistiguishable from a 'standard' space marine. Therefore, since the fictional history is by no means an authentic account but instead a collection of imperial Propaganda and the feverish ravings of traitors and heretics, the answer is, strictly speaking, rather moot.