Rich Text field with Large Image: Resizing the Image to Fit

This is a manual entry text field right? You could try and create a formula-rich text that you only show on 'View' that does a SearchAndReplace() on the rich-text field and swap out the <img tag with <img style='width: 50px' or something like that - basically trying to override the saved image tag someway. The actual img gets stored as the base64 string so it looks like <img src="data:image/png;base64....." so you would by inserting the style tag essentially. 

This would only work on view though - if you wanted it more permanent you could consider trying to do the same thing with a pipeline or webhook to do the same concept but actually save it with the style in the tag. 

I realize it's basic, but is there an option to push the responsibility back onto the user? If your user double-clicks on the image, they'll get a pop-up that allows for adding size constraints.