The sentence is, "She is as tall as I am." I'm having trouble with the "as ... as ..." structure. I understand that the second "as" is a preposition followed by a noun clause. My guess is that the first "as" acts as an adverb which modifies "tall" and that the "as I am" phrase also modifies "tall"?
She is as tall as I am.
You are correct; the second "as" is indeed a preposition; it is in construction with the first "as" which is an adverb of degree modifying "tall".
The "I am" is a comparative clause as complement of the prep "as". Comparative clauses are obligatorily reduced in some way, as your is, since the complement of "am" is left understood.
The meaning is "She is x tall; I am y tall; x=y".