vBulletin Manual - Code Block Width

$stylevar[codeblockwidth]

The code block width StyleVar is used to set the width of blocks of code in messages, as defined by the [CODE], [PHP] and [HTML] tags.

The red arrow indicates the width controlled by the $stylevar[codeblockwidth] StyleVar.

This StyleVar will accept any value that is a valid entry for CSS width. Examples of valid values include the following:

640px (640 pixels, fixed width)
75% (75% of available width)
auto (Expand to fill all available space)

Note:

This StyleVar is used as a CSS value. If you enter a width in pixels, you must add 'px' after the number of pixels desired, for example: '500px'.