Coleman LaingWere there a day MODs & achievements can coexist.
A giantess companion? As big as Liberty Prime!
The companion wouldn't be able to fit into caves or buildings.
The pieces for such a MOD are there, see for yourself.
From what I've gathered, Attack of the 50 Foot Woman [1.5] a Fallout 4 Mod By harbinger1986 only managed get this to work on the main character.
Why not a companion? The walking animation of Liberty Prime, a booming female voice actor for the giantess.