What literary element can varting sentrnce structure and length affect in a story line

Ellipsis.

Explanation:

because ellipsis is a literary resource that consists in the omission or deletion of part of the content of a sentence, which, although grammatically necessary, is implicit in the context. That is called elliptical construction.

It can also be used as a narrative resource, insofar as important portions of the content of a work are omitted (a novel, for example, or a movie), so that the reader can deduce or infer them from what is said.