What figure of speech is used in the phrase "as tall as mountain"

Simile

Explanation:

Simile is a figure of speech that compares things with "as" or "like".

In your given phrase we can see the similarity conjunction "as" is used to compare the mountain as tall.

imparadis't in one anothers arms  

the happier eden, shall enjoy their fill  

of bliss on bliss, while i to hell am thrust,  

where neither joy nor love, but fierce desire,  

among our other torments not the least,  

still unfulfill'd with pain of longing pines;  

yet let me not forget what i have gain'd  

from their own mouths; all is not theirs it seems:  

one fatal tree there stands of knowledge call'd,  

forbidden them to taste: knowledge forbidd'n?  

suspicious, reasonless. why should their lord  

envie them that? can it be sin to know,

not sure but i think it’s the snug nest