Well, I spent these last weeks trying to find a way to obtain the size measurements for the post-human of Man After Man by Dougal Dixon, the last book of the spec evo trilogy from which I could not get something with the descriptions because the focus of the texts were more focused on the situations and lives of the creatures and was less technical and informative in comparison to the other 2 books, focusing more on narrative history rather than a field guide. As a start a while ago I did an attempt to scale some of those posthumans, assuming some would possess human parts (faces, arms, legs, skeletons) that would be proportional in size to the original human beings from which they were derived, but this base couldn't work for the rest of the species of the book looking the variations of anatomy are far from the original Homo sapiens form.
Then I move on the second option, in this case scale up the posthuman based on elements or objects that interact with them that I could potentially scale and thus have something to base the rest, of course it is not so simple because of the art style and the proportions of the environment around. The image that really gave me a great help was a specific one of a Woodland Dweller hunting a duck in the tundra, this would be the initial piece as scaling up the duck (which is around 30 to 50 cm in length) I obtained the first measurement, I could eventually scale up a bunch more of posthumans based on other images that the woodland dweller share, as well using more elements on other pictures apart, but of course I didn't scale them all, just a few selected species due to this being part of a size chart for the biggest organisms, not all the organisms.
Aquamorph (Homo aquaticus): Inferred its length based on the length of its arms, considering that it is the only part of its body that is not derived in comparison to its head or legs.
Vacuumorph (Homo caelestis): their skeleton is basically a "normal" human skeleton encapsulated in a sphere, so...
Woodland Dweller (Homo glacis fabricatus): As I've explained early, I got their size from the duck in the tundra hunting scene, in an upright posture this can reach a height of 1.2 to 1.4 meters so it is not so small, just its quadruped posture shrinks it a little, reaching something like a lynx.
Tropical Tree Dweller (Homo silvis fabricatus): to have an approximate length I used the butterflies that are around, specifically a close up sketch of it holding one, being a kind of swallowtail butterfly which although is not specified the species these often have a wingspan of 10 cm.
Plains Dweller (Homo campis fabricatus): based on grass size its holding, very very speculative estimation as I didn't get the species of grass its holding and eating, but just an approach from similar grass.
Tundra Dweller (Homo glacis fabricatus): I think that the majority who have seen them imagined a behemoth of almost 3 meters high, however comparing it with the woodland dweller (specially their descendant, the Symbionts), I managed to obtain a height not greater than two meters.
Note: I also scale up with the famous "Season greetings" hunting picture too, of what I've got was a smaller Tundra dweller which fits with the description of being a "young individual" being killed.
Aquatic (Piscanthropus submarinus): there is a picture of one encased on the beach, sharing a scene with a woodland dweller so excluding angle and the distance of both I got the measure there, it is smaller than the aquamorph but it is because it is much more derived than the frog's huge legged predecessor.
Slothmen (Giganthropus arbrofagus): This is one of the few species whose size is specifically described in the book, being 3 times taller than its ancestor the Tundra dweller, and 10 times heavier, unfortunately without an estimate of its ancestor it was difficult to imagine its huge size, of course since I had achieved scale up the Tundra dweller, I could also scale up this guys, being around 6 meters tall upright.
Spiketooth (Acudens ferox): Just like the plains dweller, I based its size from the grass, althought not the size of the leaves but of soil patterns and texture (this is probably the most speculative of the scales and a little dumb but what I can do ), giving it a size of 2.2 meters tall.
It seems very small for a giant slaughter, however I considered two things that makes me think this estimation is likely and that is that the species is a pack hunting animal and that often in Dixons spec works there are always large herbivores being hunted by smaller pack hunting species.
I did more scale up on other species for curiosity but for the moment I only did these in drawing for the project.
The idea behind is that their "beast" features would allow them to survive in the long term without the need of a high intelligence r technology, something like the woodland dwellers omnivory, the tundra dweller large coat and fat, the grassland dweller and its cutting fingers and the arboreal dwellers with their prehensile hands, each one prepared to withstand the challenges of their environment
Oh yeah, I scale up that one and I got a Tundra dweller of around 1.6 meters so it was a very young one.
I really wished that they added size chart on that book :/