Flickr embeds need the Jetpack plugin with the shortcode module activated.

The flickr Gallery above is embedded by pasting in the link to the page. As is the image below:

Unfortunately there is no way to adjust the size.

Block Editor Embeds

The Block editor provides a Flickr embed block, which gives a slightly larger version of the image. If you paste a flickr url into the editor it will add an embed block or you can add the flickr block in the usual way.