Filter size? This doesn't have a filter thread. Cinema lenses typically take filters via a matte box or filter tray/clamp. The best thing for you to do is get a matte box (I recommend a clamp on) with a 114mm diameter. It'll clamp on the front no problem and you can drop in filters as you need. A good one is the TIlta MB-12 (or MB-T12, I forget) but you can get one for about 300 bucks and it has three filter stages which makes it even better.