There are 2 ways to implement images to a WPO template. The basic version is to use the “Add Image” module, which allows you to add an image directly from the Media Library, which will be automatically responsive. Height/width attributes are not added when using this method.

IMPORTANT: There are pros and cons to adding height/width attributes to images, as they are usually responsive, so there is no one single size to note.

If you do want to add attributes to images, then you should use the Custom HTML module instead. This way you have full control over the image, but have to add it manually from the Media Library, something like:

<img src="https://domain.com/wp-content/uploads/image.jpg" width="400" height="300" alt="Image">

You can also make these images responsive by adding an extra class (img-fluid), which would look something like:

<img src="https://domain.com/wp-content/uploads/image.jpg" width="400" height="300" alt="Image" class="img-fluid">

Please also be aware that image height/width attributes are a “suggestion” and not a direct part of Google’s performance factors.

In general, only metrics contribute to your Performance score, not the results of Opportunities or Diagnostics. That said, improving the opportunities and diagnostics likely improve the metric values, so there is an indirect relationship.

You may also find this helpful:
https://wordpress.org/support/article/inserting-images-into-posts-and-pages/#image-size