The enormity of the mountain range stunned the visitors to the hill station.

'Enormity' traditionally refers to the great or extreme scale of something morally wrong or evil (e.g., the enormity of a crime). For describing the vast physical size of a mountain range, the correct word is 'enormousness', which means the quality of being enormous in size or extent. 'Eternity' means infinite time. 'Extremity' means the furthest point or limit. 'Simulacrum' means an image or representation. Hence, 'enormousness' is the appropriate improvement for describing physical vastness.