You can build scanners of varying quality at systems. In the attached picture, you can see the smallest version at the Veii system and the next larger one at Capua.
The scan range is indicated by different shades of blue: The more intense the color, the more info you get about the detected fleet.
Likewise, players can invest research in stealth technology, with each level lowering the amount of info a player gains from his scanners. The quality of information ranges from fleet composition, allegiance, fleet size down to not being detectable at all.
Also, ships have a scan range, too. Scouts in particular have a pretty decent scan range, as can be seen when you look at the small ship next to Epsilon Virginis at the center top of the picture. Setting them to patrol at the edges of your empire makes quite a lot of sense. Unless you prefer surprises...
Oh, and this is a "Very Large" map, measuring 170x170 sectors. The maximum size for custom games is 1000x1000.
