Blog post thumbnail upload size (use same image file for within post and as thumbnail?)

I'm looking for a best practice for image uploads for blog posts.  I've found very clear guidance for images within a blog post, but nothing so clear for the thumbnail image to go with the post.

Can I just load the same image file to both places (into the post itself, and into blog post settings > Options > Thumbnail image), or should I make/use a smaller version (smaller file size) for the thumbnail?

Or in other words, if I use a post-sized file for the thumbnail image, will Squarespace automatically resize it (and not slow my site down)?

And is the answer the same for the Alternate Social Sharing Image?

Thanks

I use the same image file for the header in my blog post and for the thumbnail most of the time. If not, it could be an image that is posted elsewhere in the blog. Thumbnail images have the same limitation as any other image. No larger than 20mb.