Featured Images blurry after switching to block theme

I just switched to a block theme, Twenty Twenty-Three, and I’m having all sorts of problems I never had before. My first issue is the featured images are changed from 1200 x 600 images to 100 x 50 images when displayed live on the site. They are stretched out to the full size, but now they’re only 100 x 50. Also, the content area on new pages are extremely narrow. I manually changed the width for the content area in the template, but even the page title is extremely narrow. I don’t know if this is related.

Previously, I was using Themeco X theme on this site with Cornerstone. This site is only used for experimenting–thankfully.

The page I need help with: [log in to see the link]