I am considering attending MAPSS and not an student or alum so I am not sure how much this advice is worth but IMHO not knowing the topic of your thesis 5+ months prior to the start of the MA program (emphasis on MA) will in NO way make you look foolish infront of potential advisors. In fact, I know what I want to write my thesis on and the chances that I will bring it up (or at least discuss it in depth) during my fist meeting with profs during the April campus visit are very slim. Whichever professors you hope to meet with -read their books, articles, working papers, etc. and think of some insightful questions to ask -if you seem intelligent and engaged you will be no worst off (and depending on your questions/ability to charm maybe better off) than the small margin of prospective students who have a more definitive research direction.
I am also interested in class size. And campus/MAPSS culture. A lot of people on this and other forums make it seem like a sadistic hellscape where only the masochistic survive. I will have to ask about that during the Campus Days. I am sure it will go over well ; )