For everyone that likes Warhammer 40k Tabletop and Dawn of War Games or anything to do with Warhammer 40K.
Space Marine standard height: 8ft 1in
Weight: 700 lbs.
Primarch Height: 9-11ft
Weight: 888-990 lbs.
Ex: Sanguinius height: 11ft. 6in.
Sanguinius weight: 900 lbs
Legio Custode Height (with helmet): 11ft
Legio Custode Height (without helmet) 9ft 9in
Legio Custode Weight: 850 lbs
Emperor of Mankind Height: 12ft
Emperor of Mankind Weight: 1000 lbs
I think Custodes fall into the Primarch category in power and size.
It also doesn't mention Terminator armor either. Also some of the Primarchs are taller then their brothers.
Yeah, Alpharius was considered short for a Primarch as he was only just taller than a standard Astartes which was why many Alpha Legionnaires were able to 'play' him.
I think Magnus was considered quite tall for a Primarch as well, taller than average certainly.
Also Custodes are far more powerful individually than an Astartes but they are not as powerful as a Primarch (though Valdor would probably be pretty bloody close) but they fight alone even when together when the Astartes fight as one.
If you want to know more about the Astartes, Custodes and the Primarchs I would recomment The First Heretic by Aaron Dembski-Bowden. Bloody good book that one.
Sources vary unfortunately. Not just (super)humans but all the other races and vehicles (from tanks to titans to space vessels).
The famous Emperor vs. Horus art piece puts Horus way taller than the Emperor while more recent artworks puts them at about equal height.