The default screen size in the general command & conquer game is only 800 x 600 and 1024 x 768. Here are the steps so that this game can be fullscreen on your computer or laptop screen.
Open My Documents then select the Command and Conquer General Data folder
After that open the Options.ini file
then look for Resolution, after that change the value according to your monitor screen size.
Then you save the data.
Good luck 🙂