OK, WordPress will by default display the Full Size image, with a src-set
containing the other size images, the browser depending on many criteria will select the most appropriate src-set
image.
If you add an Image to a Page using the Core Image Block or the GenerateBlocks Image Block, then by default it will apply the above rule. But in the block settings you can choose the Attachment size, if you want to load a specific size image.
The only place that GP will define an image size is:
1. Activate the Blog Module in Appearance > GeneratePress
2. Go to Customizer > Layout > Blog –> Featured Images, here you can specify the Attachment size you want GP to display for Posts, Pages and Archives.