First, log into DirectAdmin as the admin user.
Locate the Server Manager section and click the Administrator Settings option beneath it.
Your path should be displayed in the top-left as follows:
Dashboard / Admin Settings /
Next, click the Server Settings tab.
Find the setting for Max Request / Upload Size. It is set at 10MB by default.
Increase this as desired and Save.
That's it! You should now be able to upload larger files via DirectAdmin's File Manager. :)