You should look at bluray & DVD approx size and what your needs are now vs future. I personally rip my blurays, full res, full audio, which usually ends up being between 25-40gb. DVDs are closer to 10gb.
I originally purchased a smaller synology 1-bay unit, then upgraded to a 2-bay, then upgraded to a 5-bay, then expanded the 5-bay with a 2-bay (7 drive array). My 7 drive array holds movies, and the 2 and 1 bay hold photos, music, videos, etc. (there sync'd).
What I'm getting at is plan ahead. Spend more now to save later.
How are you planning to store your movies?