Featured Images are showing at full size

We’ve purposely left it blank, so to speak.

Use:

.attachment-post-thumbnail {
  height: auto;
  max-width: 300px !important;
}

Change 300 to your desired width.

Or if you want it to say, float left with a margin for text to wrap around, use:

.attachment-post-thumbnail {
  float: left;
  height: auto;
  margin: 10px;
  max-width: 300px !important;
}

Please see: https://startupwp.com/topic/customizing-your-theme/

You can just add .blog in front like so:

.blog .attachment-post-thumbnail {
  height: auto;
  max-width: 300px !important;
}

You’re welcome.