Think of your door as being a picture frame for your wreath, this will help you determine the size.
Weather is always a concern. It's best to keep your wreath in a protected area. Humidity is a factor that will affect a natural wreath. For longevity keep your natural wreath in a controled enviorment.
If you have a seasonal natural wreath store it in a air tight bag.
You can use endust to clean both artifical as well as natural wreaths.