Resizing an image in a text field is easy, drag the side of the image to your desired size. However, this size does not carry over into Confluence.
Images added to a field within Jira and that field contents automatically populating in a Confluence page renders the image gigantic, making the page useless.
Here is the Jira where I added my text and image:

Here's how the content renders in my Confluence page:

See, the image is so big that it makes the page useless. Is there a way to resize the image so that it translates nicely to Confluence?