Unable to change display size of images larger than 640px in Wordpress?

Most themes (esp. themes from wordpress) have a specific width limit (measured in pixels) for images inserted in a post or a page. Obviously, that limit depends on the theme’s main column width (which varies greatly from theme to theme).

/wp-includes/media.php has:

// we're inserting a full size image into the editor.  if it's a really big image we'll scale it down to fit reasonably                // within the editor itself, and within the theme's content width if it's known.  the user can resize it in the editor                // if they wish.                if ( !empty($GLOBALS['content_width']) ) {                        $max_width = $GLOBALS['content_width'];                }                else                        <strong>$max_width = 500;</strong>

Try overriding with $GLOBALS['content_width'] = 960; in the theme's function.php

As @Debajyoti Das explains, there is a set content width for most themes.

Either find the width in your theme and amend (search the files for 640) or if you have a child theme then add the following code in your functions.php which will usually override the themes width settings.

For instance this works with the Understrap framework:

// set theme widthglobal $content_width;$content_width = 1024;