1. Some of the more successful SF maps are huge, and basicly just have tonnes of space for people to snipe at eachother with SMGs from exobidant ranges. For some reason SF doesn't like player numbers, never has. Later versions plan on fixing it. At the moment I say size is most dependent on how far away from eachother you want players to be when they start shooting at eachother. They don't need to be big to be fun.
2. Everyone does a no mans land, I don't know if there is a way to not do one. If you make coradoors it becomes too easy to restrict movement with 1-2 well placed blocks, and non noman's land maps so far have all been gimickey as hell. Personally I'd like to see more inovation used and the no man's land attraction minimized, but I don't really know a good alternitive off the top of my head.
3. I always like these, because they force more complicated fort building. Up to you, but I'd try to be a little more creative then just puting rafters in.
4. Nope, if it looks good outside is fine. SF maps have a sick characteristic of not having any immersive value at all. I'd like that to change
5. Because they once exploded one and they don't want that to happen again really...
6. I don't think people understand how source vis works. They assume that since it's a new engine they can just cram exorbidant detail everywhere all visable at the same time and it won't kill people's PC's. I really don't know enough to comment much on it when people are doing bad vis stuff.