Featured image uses wrong default size

[derweili]

Describe the bug
By default gutenberg tries to load the featured image with the image size post-thumbnail in the editor sidebar:

gutenberg/packages/editor/src/components/post-featured-image/index.js

Line 34 in c4cd4d8

const mediaSize = applyFilters( 'editor.PostFeaturedImage.imageSize', 'post-thumbnail', media.id, currentPostId );

This image size does not exist by default. The correct image size would be thumbnail.
See the default image sizes in the Codex: https://codex.wordpress.org/Post_Thumbnails#Thumbnail_Sizes

Because the specified image size does not exist, the full size is used as a fallback.
This causes a performance issue, because a potentially far too large image is loaded.

To reproduce
Steps to reproduce the behavior:

1. Create a Post
2. Add a Featured image in a large size e.g. 2000px wide
3. Right-click on the image and choose "open image in new window"
4. You can see the image in full size.

Expected behavior
The Featured image should be loaded in a smaller version/thumnail site within the editor sidebar.

Desktop:

• OS: OS X 10.13.6
• Browser Chrome
• Version 74.0.3729.169

Additional context

• Gutenberg Version 5.8.0-rc.1

[derweili]

I think there should be a discussion which image size should be used in the editor sidebar.
Maybe medium is better than thumbnail because it's not cropped by default but i think that neither post-thumbnail nor the full size should be used by default.

[senadir]

WordPress uses medium size by default in the old editor so I think this should be respected in gutenberg as well