Image Size wrong during upload

The maximum width of an image displayed in content is not just determined by the image sizes you set or CSS settings, it is also determined by the content width set in functions.php through code similar to the following

if ( ! isset( $content_width ) ) {
    $content_width = 960;

You need to set this to your required size as well in order for the image to display correctly with this bigger image width. Without setting that accordingly, even if you set your image sizes correctly, the biggest an image will be in the content area will be 960 px as in example.

You also need to remember, images can be up-scaled, so any image smaller than your new size will only be as wide as its original size