Also remember that VERY large outside areas should actually be many seperate smaller boxes.
For example, you come out of the house and into the garden. There is a high wall around the garden. There is a gate in the wall. You go through that gate to another outside area.
You can make the two outside areas completely separate areas, each with their own sky box. This allows you to cut vis way down and keep your map running smoothly. Here's a really good tutorial to look at: http://www.snarkpit.com/editing.php?page=tutorials&game=HL2&id=143
I don't recommend that you build one huge skybox around your map while working on it. This will cause the engine to render a whole bunch of stuff that it doesn't need to. It will also throw off your estimates when you go and check your map in-game.
For example, you're thinking of adding a bunch of detail, but when you look at your map in-game, you are already eating up most of the budget. This is because the engine is drawing a whole bunch of stuff it doesn't need to. Adding detail then becomes a guessing game because you won't really know what your budget is until you fix your sky boxes.
Not to mention if you build everything inside one big skybox, and then go back to try to section it off, you may find places where it doesn't quit fit and you have to go back and rebuild stuff to get your skyboxes fixed. And then you could end up with leaks, and you'd have to chase them down which is never fun.
The best policy is to make a "best guess" at how big your emmediate outside area will be, and build the map so that area is a seperate section, with it's own bit of skybox. Make sure it is sealed, build your garden, and then go on from there. If you need to expand your garden, just stetch the walls, floor, and ceiling out to accomidate your needs. keep it tight and sealed, and that way, you never have to go back and do "clean up".
Good luck,
D-Gen
