Changing Featured Image Size

Hello.

Is there a way for me to resize the featured image size on my posts? I can’t seem to figure it out. Eg: https://www.bomarrblog.com/a-fleeting-tweet-brings-illogic-and-odd-nosdam-back-together-after-23-years/

I’d honestly be fine not having a featured image at all, but that seems to be the only way I can have a thumbnail for the post appear in the sidebar of my site.

Am I missing something completely obvious here?

Thanks!

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