Go to your Freestylegunz folder, (dont know where located at your pc)
Search for config.xml, open it with notepad.

Replace your old video settings, with this one.
<VIDEO>
<WIDTH>640</WIDTH>
<HEIGHT>480</HEIGHT>
<COLORBITS>8</COLORBITS>
<GAMMA>255</GAMMA>
<FULLSCREEN>TRUE</FULLSCREEN>
<REFLECTION>false</REFLECTION>
<LIGHTMAP>false</LIGHTMAP>
<DYNAMICLIGHT>false</DYNAMICLIGHT>
<SHADER>false</SHADER>
<CHARACTORTEXTURELEVEL>2</CHARACTORTEXTURELEVEL>
<MAPTEXTURELEVEL>8</MAPTEXTURELEVEL>
<EFFECTLEVEL>0</EFFECTLEVEL>
<TEXTUREFORMAT>8</TEXTUREFORMAT>
<NHARDWARETNL>false</NHARDWARETNL>
</VIDEO>
You can edit WIDTH and HEIGHT to
<WIDTH>1024</WIDTH>
<HEIGHT>768</HEIGHT>
For a bigger screen, save & close now.

By the way, if you edit something ingame options, your config.xml wil be resetted and you have to do this steps again..

Greetings,
Weezy