Reference anywhere for SharePoint Modern page image sizes?

As a product designer on the SharePoint team, I too found it odd not having the image sizes published anywhere. But don't fret - I can provide some details, but if you're looking for pixel dimensions well that will depend on the screen size, resolution, location of the image asset in a page section, ect. That latter part is where SharePoint's adaptive handling of content resolves and help you create great layouts.

TLDR; Using a 16:9 image ratio and setting the focal point will help maintain the correct perspective across all breakpoints. Images used in heading don't have to be your news image thumbnail. You can add an image to heading background, then set your news thumbnail in "Page details". And you guessed it - 16:9 works best for this as well. You can even update the descriptive text here too - making your metadata/open graph information exactly what you'd like.

SharePoint uses an adaptive layout where the system managed the reflow and sizing of elements. Images are placed in their elements to "fill" so that they too scale maintaining perspective of each items' height and width. Now while we recommend 16:9 ratio - we also recommend you optimize (compress) your images since file size has a huge impact on page load time. There are tons of great and free image compressing products across the web. And of course - don't forget - alt text. Even if that image is meant to be decorative, add that so that your pages and content are accessible.

+1 on the adaptive handling of assets most people are referencing to in the comments. It's difficult to provide a pixel perfect size for image assets because the scaling of elements to provide the best UX across different screen sizes. That said, there is this documentation we put together that may provide more information: https://support.microsoft.com/en-us/office/image-sizing-and-scaling-in-sharepoint-modern-pages-dc510065-b5a5-4654-bc94-e3ecbbb57d8d

I know this is an old thread and there is an accepted answer, but the OP question was not specifically about creating a home-page mosaic. My issue was with news posts thumbnails, and my search led me here. To answer the OP question, however, there is an [support article on the subject](https://support.microsoft.com/en-us/office/image-sizing-and-scaling-in-sharepoint-modern-pages-dc510065-b5a5-4654-bc94-e3ecbbb57d8d#:~:text=By%20default%2C%20the%20thumbnail%20comes,a%2016%3A9%20aspect%20ratio.).