Yeah, what they said

Obviously I was thinking of sensor range...
AFAIK, when making your ship, take the range value (usually .2) and add it to 1. That should be the number of sectors(grids, or the larger grids if you have the grid feature on), beyond ones that contain your colinized planets, that your ship can travel before running out of "range"... Like they said, increase this value with life support modules. Not neccesary on small maps.
Not sure if the range value refers to the range from any of your colinized worlds, or just the one that it was manufactured at, as I usually play on small sized maps. But I believe it's the former, although, for some modifiers, the ships do keep track of where they were made...
EDIT: I'm pretty sure starbases can also be used as a "base" to increase the range of your ship.