Online peer assessment on Facebook has been proved to be a successful technological tool for supporting English writing classes thanks to its effectiveness to students’ learning performance and learning satisfaction and processes (Chang & Chen, 2009; Lu & Bol, 2007; Nurlia, 2014; Shih, 2010; van Gennip, Segers & Tillema, 2010; Xie, Ke & Sharma, 2008; Xiao & Lucking, 2008; van Zundert, Sluijsmans & van Merrienboer, 2010; Wichadee, 2013). The implementation of peer assessment tends to be less likely to achieve fruitful outcomes with classes of large size because of certain challenges confronted by both teachers and students (Agaron, 2003; Hron & Friedrich, 2003; Qiu, Hewitt, & Brett, 2012). In English language teaching, however, few studies investigate possible impacts of class size on the effectiveness of Facebook peer assessment in English writing classes. Thus, this gap is explored in the current study. In particular, the study employs a qualitative case study approach and utilizes data from the survey with 107 students and the interviews with two English lecturers of three classes of English essay writing for English-as-a-medium-of-instruction (EMI) classes. The results reveal there are certain impacts of class size on the implementation of Facebook peer feedback in EMI classes in terms of its merits and drawbacks. Based on the findings, possible recommendations are then proposed for further successful implementation of online peer assessment on students’ Facebook groups especially to large classes.