Turns out, based on the responses, that the topic is much more complex than I first envisioned. One alternative is for someone to edit the corresponding Help article (referenced below), with a link to this article.
Then, the user can read the full details about the issue of image sizing. In the meantime, whenever I reference the corresponding Help article, I will also reference this article.
The Images section, at the bottom of this article, documents how to embed images into a mathSE question or answer. I recommend enhancing that section of the article to include something like the insertion in this posting, that follows the Edit section:
Thanks to Ray Butterworth's answer for an idea that I totally overlooked. I have edited the suggested insert (below) accordingly.
Actually, I don't know what to make of his response. I am assuming that if no width parameter is provided, then the default will be whatever the pixel-width of the uploaded image happens to be. My guess (which could be wrong) is that if you are using a middle-of-the-road display resolution (whatever that means), then it would generally be best to adjust the width until it looks suitable to you.
Re the article referenced in Calvin Khor's comment, in my opinion, specifying absolute pixel size seems to give the most flexibility. Therefore, the only concept that I took from the article for insertion into the suggest edit (below) concerns optionally changing the aspect ratio.
Start of Suggested Insert
You also have the option to control the size of the displayed image via the following syntax:
<img src="https://i.stack.imgur.com/m2uYu.png" width="200">
When formatting your posting, you can experimentally change the width parameter above to find the image size that you regard as the most suitable for your posting.
You can also specify the height only (instead of the width). Further, if you don't mind changing the aspect ratio change, you can specify both the width and the height:
<img src="http://i.stack.imgur.com/m2uYu.png" width="300" height="500">
Be aware that, in general, using absolute sizes can produce something that looks good on your screen, but awful on other people's. For example, to someone with a 4k monitor, a 200 pixel image will look tiny, since it is only 5% of the width of the screen. However, to someone using a low resolution monitor, the image may be seen as huge, depending on just how low resolution the monitor is.
End of Suggested Insert
My motive in making this recommendation is that when I post a comment that refers a user to the corresponding help article, I will no longer need to supplement my comment with the additional instructions that pertain to controlling the size of the image.