In Fancy Product Designer, it is possible to show the image size in the designer. In order to do that, you could navigate to Fancy Product Designer > Settings > Advanced > Miscellaneous, then enable the Size Tooltip option:
Once you enabled that option, in the product designer, you should be able to see the image size as follow:
The size tooltip will works in any element, including the text element:
You could change the unit of measurement in the Fancy Product Designer > Settings > General > Actions:
