well, 40k is roughly scale 1:60 and epic is in 6mm roughly 1:300.
if you thought about using epic modells for 40k you can forget about it. it's way off scale. the cobra you're speaking of is about 5cm long. remember, epic is for mass battles. meaning whole marine chapters and loads of titans per battle. an emperor titan here has a height of about 23cm. and it's by far the biggest modell in the game..