Optimum Photo Size

900 is fine.
Just make sure you compress it first :
https://tinypng.com/

Your entire width is defined at 1140px.
And the blog renders the same layout format as your main page :
https://ibb.co/J7rGWXq

It sort of depends on your Media Settings and on your theme. The featured image is only used by the theme (okay, plugins do stuff also, but mainly theme), and the theme can choose the size to use.

It makes sense to me that the theme would simply use whatever sizes the user chose for the Media Settings, and the size the image was when uploaded, but most themes do not.

Please don’t focus solely on the desktop size of 900px. Users will have quite a range of device and browser window sizes, including small phones and super large monitors, not to mention Retina screens which need a 2x version to look good.

Choose a file size instead, so that bandwidth isn’t wasted, and your server isn’t clogged with huge files that download slowly. As long as the file size is 300K or less, and the image isn’t pixelated, the width doesn’t matter so much.
Besides, you might change themes some time.

The ideal size is 1,200 pixels wide by 675 pixels tall (or an aspect ratio of 16:9). The minimum size is 600 pixels wide by 335 pixels tall but larger images will look better when your followers click to expand the image.

Thank you all for your replies. I appreciate them and they are helpful.

@corrinarusso. Thanks. I do use the “save for web” feature in Photoshop to compress my images. I’m guessing that works well enough. Maybe not?

@joyously, are you suggesting I should use files larger than 900 px on the page or use smaller sizes to meet the needs of different devices? As for the featured image, not only I am concerned about how the theme uses it, but what about social media sites? Isn’t the FI the image social media sites display when a link is posted? If so, is the resolution of the FI that appears in social media also controlled by the theme or do social media sites use the actual size of the featured image?

@calliealbert, I have heard that is the size (or something close to it) is what Facebook prefers to use when displaying images. Thanks.

I am suggesting that it doesn’t really matter what width your image is. Larger width originals produce smaller versions that are used by smaller devices (chosen by the browser due to srcset attributes added by WordPress). People that have Retina screens need a 2x version, but unless you have a theme or plugin that does something about it, they won’t get that. That’s why I say to focus on file size so the original can be used without the file being slow to download.

The image used by social sites is determined by the external site, not WordPress. You can use a plugin to add the opengraph tags or other markup to the header so that the external site knows which image to use. See https://yoast.com/advanced-technical-seo-social-image-ogimage-tags/

Thank @joyously! (BTW, great username)

I appreciate the information and the link. So 300k or less is good and that should be my focus and not the pixel width.

Thank you!

Pat