Right click on the shortcut that you normally use to start Freelancer then click Properties (Shown by the arrow) like so;
You will get a form showing this;
As the above image shows, use the Target field (Shown by the arrow) to add " -dx" to get the game to automatically go to your Desktop resolution, so it will be the same size as what you use for the desktop. You can also add it in manually like in the picture such as " -d1920x1200", replacing the numbers with your desired resolution. Remember to have a space between the end of the shortcut target and the start of the widescreen parameters, like so:
The following command line options can be added to the shortcut.
-dx
Widescreen support, where the screen size is set to the current desktop resolution -dWxH
Widescreen support, where W = screen width and H = screen height. -noflighttext
Hide the "Mouse Flight" and associated text on the hud. -nohud
Disable hud -logchat
Log chat to My Documents\Freelancer\dsace.log -logappend
Do not delete the dsace.log everytime freelancer is start. Append new chat to the existing file. -logtime
Include the timestamp in chat log -localtime
Show the timestamp in local time rather than UTC. -newplayer
Enable new player messages. By default new player messages are disabled -dptplayer
Enable departing player messages. By default departing player messages are disabled -lag
Show the lag indicator icon. By default the lag indicator icon is disabled.