You will either really hate or really like this one. Not for the easily distracted, Universal Combat comes from the BattleCruiser series by infamous Derek Smart. Published in 2004 and released as freeware in 2008, it is a FPS, naval sim, as well as a space combat sim all rolled into one. (Several of the previous iterations in the series can be downloaded from the link below, including BattleCruiser 3000AD and BattleCruiser Millennium) Approximate size for all three games: pretty big.
Download Link: http://www.3000ad.com/download/