They published two different versions, a Professional and a Standard edition. Minimum install takes 420Mb of your HD, and maximum install demands a hefty 1.3GB of hard drive space.
Installation is pretty straightforward.
You choose where to install and what you want to install. Scenery is the largest consumer of hard drive space, and it is a Good Thing that Microsoft lets us choose which sceneries to install and which not to. Remember to click the box for internal views if you want to see the interior of the aircraft when you look around in the cockpit. In FS98, interior views consisted of textured 3D views of the actual 3D model of the aircraft. This made it possible to see the control surfaces moving on the wings etc.