Apologies for the delay in responding.

Updating of documentation is an ongoing process that has to fit within development and support schedules, one of the challenges of running a 1 man operation! I hope to have most of the Image Manager Extended docs updated by next week.

I'm stuck trying to find how to enter the Source and what sizes to set for Responsive images

When in the Responsive tab, click into the first Source field, then select an image to use for this field. Check each subsequant image to include, and a new Source field will be create for each.



For the sizes, you need to decide if you want to use the Device Pixel Ratio or Width Descriptor, and set the relevant W or X values for each image accordingly. This is explained in the documentation - https://www.joomlacontenteditor.net/support/documentation/image-manager-extended/responsive-tab

First, you need to add the following to your template stylesheet:

img {
  max-width: 100%;
  height: auto;
}

This will ensure that images are resized responsively when the browser window sized is change (resized, orientation change or device size change) Looking at the code for the image on your site, it does not look like Responsive image was applied. The srcset value should be

 srcset="https://www.joomlacontenteditor.net/images/home-images/little-warham-cottage-living-room-welcome-2.jpg 2x"

[quotePost id="113557"]There's nothing I've seen in the JCE documentation or the embedded online Help for the component that mentions a template override. Thanks for the information, though.
Prior to the Joomla 4 conversion from Joomla 3, I used a CSS setting built in the RocketTheme template ('img-responsive') to tag images. Seems that tag no longer works.[/quotePost]

It's not really a template override, just an addition. Most template stylesheets already include the above css as it's a pretty standard thing now for responsive images.