@jasonekratz This is actually something you can accomplish in your custom CSS within the Micro.blog design settings (no custom theme or plugin needed). In Custom CSS, just add:

img { max-width: 600px; max-height: 600px; width: auto; height: auto; }

The 600px number is derived from how Micro.blog handles images. The HTML always includes 600px as the dimension of the widest part of the image (height for portrait, width for landscape). This will be the size within the img tag unless you've further modified your site like this.

Let me know if that gets you where you want to be.