By default, IPS4 has a maximum body width of 1340 pixels, and is fluid at sizes smaller than this. However, we ship the software with a customizable theme setting allowing this to be easily overridden, should you want to change the behavior.
Using the Easy Mode theme editor
If your theme was made using the Easy Mode editor, you can adjust the body width on the Settings tab when designing your theme. First, click the magic wand icon to launch the Easy Mode editor:
And then the Settings tab. There's two settings you need to adjust; turn on Enable Fluid Width, and then change the percentage value in the Fluid Width Size setting. To have it take up all available space, set this to 100%.
Using the standard theme editor
If your theme was created using the standard theme editor in the AdminCP, you adjust the theme settings also in the AdminCP. Click the edit icon next to the theme you want to adjust:
And on the Custom tab on the edit screen, you will see the Enable Fluid Width and Fluid Width Size settings.