You can choose the exact dimensions by using BBCode (the actual text formatting used by XenForo in the back) instead of the rich editor (which just converts to BBCode for you behind the scenes). You can do this by using the following formatting:
Code:
[img width="<number>px" height="<number>px"]URL here[/img]
Note that it will adhere to the original aspect ratio (if I remember correctly), so it may not obey your second argument (in the example above, height), if it would cause the image to be stretched/compressed.
Here's an example image resized to a width of 75 pixels:
The BBCode for that is:
Code:
[img width="75px"]https://s3.blankdvth.com/57a9bca1-5fc0-4f4d-9821-9d53bfa3b993.png[/img]
You can also use percentages, by using something like
width="50%" instead of
width="75px"
Also as Luna has said, using direct links to images hosted by other sites (known as hotlinking) is bad practice/bad etiquette and is generally discouraged.
This post explains it decently well.
Side note, I can't remember if you need to exit out of the rich editor and use "BBCode mode" for BBCode syntax to work, I never use the rich editor (I always just use BBCode plain with the needed tags) so I can't remember. If you do need to, this can be done by clicking the button that looks like two square brackets (
[]) in the top right of the editor bar. It may be hidden under the dot menu (three vertical dots) if you have a narrow screen.
I won't change the status of the suggestion since I'm assuming it's for adding this functionality to the rich editor, but just know that it is possible.