Hi there,
To change the size of the featured image (for all posts) you can go to the site editor, and locate the “single” template and edit it.
You should be able to find it at this URL https://localdemocracy.net/wp-admin/site-editor.php?postType=wp_template&postId=pub%2Fblockbase%2F%2Fsingle
If you click on the list view (1) then the group block that is holding all the post content (2) you can set a max-width for that group. You can then set it to 600px, which is the width of the text column.
See this link for a screenshot of what I mean above: https://d.pr/i/Ktou2O
Hope that helps. Please let us know if you have any more questions.
