。
The eyelids open wide.
Literal
Eyelid [subject-が] big-[adverbial-に] opens.
A descriptive narrative line. 瞼 (まぶた) 'eyelid' — a specific anatomical term. 大きく is the adverbial form of 大きい ('big'), modifying 開く ('to open'). The plain dictionary-form ending gives a narrative present-tense feel common in stage directions and descriptive writing.