Sort of related: I JUST started reading Unremembered Empire, and realized on the cover that Guilliman and Sanguinius are attended by tech-something or other. Holy crapo, they barely come up to the Primarchs' waists. Are the Primarchs supposed to be the size of two-story buildings?
I thought Space marines averaged 8.5 feet tall, in armor, and Primarchs were 12-feet tall or so.
Magnus is the tallest, especially so, and he stands at about 10.5-11ft.
A space Marine is on average 7.5-8ft tall and a primarch is on average 9-10 feet tall. Considering that some humans have grown as tall as three meters in height due to mutations, it's not that far fetched.
http://www.lounge.belloflostsouls.net/showthread.php?52423-The-Blood-Pact-Chaos-Homebrew-Supplement&p=472214&viewfull=1#post472214
And those don't seem to be techmarines attending them, so it wouldn't be unusual for them to be less than even 6 feet tall. Waist height seems about right.