You can use
m for small and medium image size respectively before "." in your image URL.
For example, if the image URL is like
https://i.stack.imgur.com/6touk.jpg and you want to keep it medium size, add
m before dot
This post from Main Meta SE explains in more detail.
Where the original might be something as obscenely large as this:
Slip an m or s character at the end of the file name, just before the file extension, like so: