Micro.blog offers an endpoint to resize uploaded images. Once you upload the image, if you do https://micro.blog/photos/###x/, but replace ### with the pixel width you want, followed by the full URL to the image, you’ll show an image with that width. For example, if I wanted the image at https://json.blog/foo.jpg to be 480px wide, I could use <img src=“https://micro.blog/photos/480x/https://json.blog/foo.jpg”>.
I have built that into the glightbox plugin that I maintain, so that images will be resized if they are put within a gallery. You can see these thumbnails in action on this post.
As for whether there is a standard size, I think that depends a bit on your theme.