[Resolved] Archive's Featured Image Size

I uploaded 1200 x 800 featured images so I could also use them as the first image at the top of my blog post with high quality. I didn’t want to upload a small image and then have CSS expand it causing blur. However, when I run a GTmetrix test, one of the recommendations shows that I should upload smaller images of about 300 x 200 to prevent HTML and CSS from reducing the size of the image.

How can I tell WP Short Posts to use my medium size 300×300 images set up under Settings/Media? I understand that WordPress creates different image sizes and uses the most optimal size, but I don’t think it’s happening in this case. How can I do this?

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Thank you,

Rio