Difficulties setting a maximum image width

Currently I have this written in my functions.php:

if ( ! isset( $content_width ) )
  $content_width = 560;

If I understand correct, this should re-size a relatively large image (1024x1024px) to an image with a maximum width of 560px.

My images however are getting re-sized (to a width of 800px), but not to the specified value of 560px.

Does someone know what I might be overlooking here, could someone explain to me the use of $content_width, and where the value of 800px currently is being specified?

For what it’s worth all of my values listed at options-media.php are set as when installed.

Best regards,

Jen