I will be going off of the followingrules/assumptions;
-A default, unedited vanilla male player is an average man of 6ft.
-I will ignore editing of flyers for the following reason; In accuracy, only Quetz could hope to be ridden, and having to wait untill that high level would turn many away from Milo's mod as flyers make the game easy and are a staple to any player.
-I will also ignore creatures that are specifically modern animals such as the 'Snail' and the 'Manta' et cetera for the following reason; They are supposed to be 'super evolved'/'hybrids'/whatever excuse the devs can pull out of their rumps. I will not edit them as they are not quite certain when it comes to what they are by genus.

Now... I will also justify many creatures(will probably justify all of them by the time I am done.)
They are not currently in order but that shall be arranged with time.



Carno= 70% vanilla size
(If you compare a Carnotaurus ingame to a human player, you quickly realise the damn thing is 40ft long, approximately the length of a large Tyrannosaurus irl)
Rex=52.5% vanilla size
(Going off the famous T. rex Sue at 42ft, slightly larger then vanilla Carno, and if you look in-game you quickly make the approximation that the T. dominum is double the size of the Ark Carno, putting it at 80ft)
Gigant=28.125% vanilla size
(Gigant is ~double the length of the T. dominum(80ft) meaning a monstrocity of 160ft(bigger then Argentinosaurus, the largest dinosaur with existing evidence *shuns Amphicoelias fragilimimus*. This means that the 45ft irl Giganotosaurus is tiny af compared to the ingame monstrocity)
Megalodon=100% Vanilla size
(If you compare to the lower estimate of 52.5ft, Megalodon fits in nice and snuggly)
Dunkle=40% vanilla size
(The Dunkle in Ark is around the size of the Megalodon; IRL they were only tiny things of 20ft)
Liopleurodon=79.2% vanilla size
(This one is quite close because the IRL Liopleurodon ferox is of 21ft and in-dossier(not yet available in game and devkit is unreliable) is of around 25ft)
Ichthy=100% vanilla size
(Surprise surprise, Ichthy is of correct scale)
Mega Piranha= 55% vanilla size
(This was rather easy. The ingame is around as long as a player is tall when IRL they were only 3,3ft)
Trilobite=77,8% vanilla size
(This was quite odd. The ark devs went with an entire Class... So I went with the largest genra in Isotelus rex because most of them are excessively tiny and I wanted one that you did not have to squint your eyes to see ingame. This, a 28" organism, compared to the ~3ft ingame made my life a heck of a lot easier.)
Triceratops=100%Vanilla size
(tfw everyone says "ermagerd trke is 2 smol hes just a bby!" Turns out it is of correct stature. It is just T. dominum the oversized bastard.)
Baryonyx=62.5% vanilla size
(It is quite close in length to the Ark Carnotaurus, so I just use the data I got on Carno to shrink it down to a nice 20ft of the real animal)
Gigantopithecus=100% Vanilla Size
(Another of those lucky enough to not be required to shrink! It is not quite as large as the biggest of the species of Gigantopithecus irl but quite comfortably in-between that of the two known)
Gallimimus=74%Vanilla Size
(This was tricky. I had to use the height of the Galli ingame and compare to that of a irl Gallimimus. The irl's had an extra 50% of a human in height, but in Ark they have an extra 100% of a human in height. I simple did the proportions and got, compared to the irl length of 20ft and ended up with a length of 26,7ft in the Ark specimen. The rest follows naturally. :) )
Ammonite=183,3% vanilla size
(AAARGH. This was beyond annoying. You see, I was trying to compare the Ark's Ammonite to a proper genus. Problem; The ark devs literally chose and entire Suborder... Then, when I tried to find out if Parapuzosia, the largest ammonite, was in Ammonitina. Google searches did not help, and on the Wikipedia page it only lists the "Kingdom/Phylum/Class/Subclass/Genus/Species" and not the Suborder, or even the freaking order! I had to dig through every Superfamily... But I found it! Turns out, if the unknown Ammonitina is a Parapuzosia, it is undersized compared to both the small and largest sizes. So i chose the largest naturally. Finally an undersized creature!)