The software allows you to edit the width and height of images after they are inserted into a template, but you cannot edit the file size once it is uploaded. To keep your emails opening quickly, it is recommended that your image file size be less than 1 MB, and, ideally, the smaller the file size the better.  

For pixel width, the software automatically fits the image into the column width. The default column width for the whole template is 600 pixels, which is standard for most emails so that images show clearly in email clients like Outlook and Yahoo.  You can stretch the width up to 1200 pixels if desired. See this article, "How do I create and edit an email template?" for more about editing the Container of your template.

The software will also automatically scale the pictures for mobile viewing, which you can see using the Device preview drop down in the upper right corner. 

Using the Setting function, you can adjust the Width and Height of an image, just keep in mind that if you change one from its original size, it may stretch or shrink the image out of shape. Adjusting just the Width will maintain the original ratio, so you would not need to adjust the Height as well.

If the change was just made, you can click on Undo (counterclockwise arrow) to return to the original size, use the small white arrow keys at the end of the width/height boxes, or type in the desired width/height to adjust the size.