In the support document in Jetpack.com
It tell us we can add this to themes functions.php
if ( ! isset( $content_width ) ) $content_width = 800;
In Media --> Image sizes
I had set the Large size image is 1024x1024
but the image will resize into 800 as the $content_width was 800, the setting in Image sizes will be ignored.
How to fix this problem?