We know that there are ten times as many Sranc roaming the Ancient North today as there were during the First Apocalypse. Yet Sranc are always depicted as male, so would appear to be incapable of mating with one-another.

There is also a Sranc who says, in reference to the Old Fathers: "Father… father! We have no fathers save the earth!", which suggests the Sranc somehow gestate within the earth (like Orks!), but I seem to remember somebody mentioning a reference to female Sranc, so perhaps that is wrong.

It is possible that all Sranc spill out from Golgotterath, but if that is the case how are there wild Sranc clans who don't even know who the Inchoroi are?

Before Arkfall the Inchoroi seemed to be a lot larger than they are now: Sil's shield is the size of a small boat and he wore dead bodies as armour, and we know Aurang used to be a lot bigger -- he is capable of wrestling the Tall, and he explicitly states that he used to be monumentally tall before all the grafts somehow made him man-sized (or 1.5 man sized).

So it stands to reason that the Heron Spear was likewise gigantic, yet it seems to be something a lone man, Anaxophus, can wield. Moreover its sister spear is used by both a nameless Nonman and by Kellhus.
Does this mean that the Heron Spear is actually a lot larger than one would guess, or was it always man-sized something like a laser pointer in Sil's hand, something that fit in his palm?

I don't think anything in the text contradicts the idea that it is a lot larger than we assume: it was stolen by Seswatha and Nau-Cayûti together -- perhaps this is the reason Seswatha needed Nau-Cayûti to come with him -- and was transported in Anaxophus' chariot, so it could actually be ~20 ft long.
Then again there is nothing to contradict the idea of the spear being like a laser pointer, other than the fact that it is a silly image.