choosing undergrad dissertation topic

i am doing a dissertation in my last year of a Bachelor's of english. i am having a problem in what to choose and how to choose my dissertation topic. im leaning towards something related to feminist literature or dystopian literature but I can't seem to figure out how to narrow it down. i was thinking of something related to ecocriticism paired with feminist literature. Apparently, our dissertation is supposed to be about 15,000 words long. al th eoption i have thought of have been very vague so i would appreciate if i could get some answers for this.