Hi @coinnewsweb,

This comes from the active theme. below is provided a CSS solution you can use just change the value of the width property as you like: 

body.center-content #primary {
    width: 86%;
}

Put the CSS code in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Style Tab.  

Please don't forget to delete all caches and press Ctrl+F5 (twice) on the frontend before checking to reload the updated CSS file.