1. You can adjust the width of the template by overriding the __site_width__ variable.
2. Simply create the file lib/tpl/dokuwiki/style.local.ini and set the variable accordingly.
3. You can either use a fixed value (px or em) or a percent value (%) if you prefer a dynamic width.

Example:

[replacements]
__site_width__  = "100%"

Other available options: https://www.dokuwiki.org/template:dokuwiki#changing_the_width_and_other_styles

Note: Go to Configuration Manager and click on the Save button to make the changes effective.

New version

1. Go to Admin->Template Style Settings.
2. Set The width of the full site (can be any length unit: %, px, em, ...) = 100%.