Generally I prefer 8 or less dorm rooms. 20+ could be interesting though.
"Ensuite" means there's a shower and toilet (WC) in the room. Some hostels have a curfew, meaning you're shit out of luck if you show up past whatever it is (2am is standard). If you're planning to party all the time, make sure you get a hostel that advertises no curfew. I've stayed at the Ideal Youth in Barcelona which was pretty nice, and I've heard good things about the Kabul but they do not book online-- you just have to show up there. For Paris, I've stayed at the Square Caulaincourt but I've heard very good things about the BVJ Louvre, Le Village and the Woodstock. Avoid Peace & Love, 3 Ducks, and the Young and Happy hostels in Paris.