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: