Plugin is not using the image size I set

I have been using Custom Related Posts plugin on one of my websites, and its been working great (Thank you to the dev!) up until recently when I noticed that the custom image size I had setup using simple image sizes plugin is no longer working in my related posts output.

I tried setting a 300×200 size in the image size option of the plugin but that also did not work for me. Can you please help me resolve this issue?