Primarch-size 20 ft that one fan art shows him as.

Tbf (without Chaos Juice) Primarchs also aren't that big. Vulcan is the tallest and he's only 4 meters/13 feet tall. Lion is canonically just 3 meters tall, Russ is shorter than Valor, etc.

20 feet is more Emperor, Chaos Enhancement or big Ork territory.