Sometimes when I answer a story ID question I'll include a picture of the book cover, typically grabbed from Goodreads or some similar site. I generally try to choose what seems a sensible size for the picture, but I have had people edit my answer and resize the picture I used. Is there any standard for the ideal size to use for a book cover in an answer? If not, should there be?
No, there is no preferred image size for story id answers or for any post for that matter. Use whichever size you think makes sense in context to how you are using it.
There are no rules on it and nor should there be, in one case a huge image might be warranted in another a tiny image necessary, we shouldn't enforce rules on it.
That said personally myself I will leave it alone if the image is landscape, on its own between text and there aren't too many images in my post. Else if the image is portrait, I have a few images or a few images together I go by gut feeling for how big is too big and if it is too big downscale it using imgur's modifiers. I tend to pick
m and set the click through link as the full resolution image. For example:
[![This is a test image]] <sub>_Click image to enlarge._</sub> : https://i.stack.imgur.com/1X4XDm.jpg : https://i.stack.imgur.com/1X4XD.jpg
It can be quite awkward to do if you have a lot of images in the post but personally I have gotten quite quick at changing it. Just a matter of copying a few lines and changing a couple of numbers. For a list of other imgur modifiers see this answer.
It is worth noting if you want to do it all inline you can use:
[![This is a test image](https://i.stack.imgur.com/1X4XDm.jpg)](https://i.stack.imgur.com/1X4XD.jpg) <sub>_Click image to enlarge._</sub>
This renders the same as the above but depending on the individual may be harder to read.