I voted for 25 man since I like the larger size due to feeling more like a raid instead of a dungeon with longer fights. If they were to compromise and make a single raid size I would prefer 20man. IMO 15 would not be large enough to feel like a raid while 20 would be only a little noticeable drop from 25. It would also probably be easier for guilds to conform to a 20man roster since 10 man guilds can merge and 25 man guilds can let go of their 3-4 low performing members. In 15s a 10 man would have to recruit 50% more people from split up guilds and 25s would have to almost split in 2, it would be very disrupting.