Size of post image

Hi there,

to do that in the three theme is going to require some code.
The simplest method is to add some CSS:

/* Set max size post image on desktop / tablet */
@media(min-width: 760px) {
  body:not(.single) .post-image {
    max-width: 250px;
  }
}
/* Force post image full width on mobile */
@media(max-width: 768px) {
  body:not(.single) .post-image img {
    width: 100%;
  }
}

I don’t think that plugin will help.

You can either use David’s code, or we also have the featured image resizer options in the premium plugin:
https://docs.generatepress.com/article/adjusting-the-featured-images/