Valmiki Ramayana Sundara Kanda Sarga 34 Shloka 25

सीता Sita, इत्येवम् in this manner, बलाबलम् strength and weakness, बहुधा in many ways, सम्प्रधार्य after considering, रक्षसाम् of demons, कामरूपत्वात् taking any form at will, तम् him, राक्षसाधिपम् king of demons, मेने he thought.

Considering the strength and weakness of demons in that manner Sita thought that Hanuman was none other than the king of demons who can asume any form at will.