in the softlight future... there is only cake

This is how awkwardly an astartes would bend down to be face to face with you

and this is how you'd appear from his point of view

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This is you next to an average primarch 335cm (10'9)

Practically the height of his knee. You'd struggle to reach his hand alone.

prime kickable height. A primarch could easily trip over you without looking

how you'd look like from his point of view

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This is you next to an average chaos infused Horus 433cm (14'2)

The equivalent of a family size bottle of coke. His claws could repurpose you into a squishy

roughly reaching midway to his knees... he has unusually low knees

What you look like from his point of view

Arguably you could use chaos Horus's height as a substitute for big E's

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Bonus:

Watcher next to primarch

Astartes next to primarch

Astartes next to Chaos Horus

Pelvis height