masonry widget: Responsive images/proper image sizes

[strarsis]

Currently the masonry widget uses images in full size or very large size, much larger than necessary.
This dramatically increases bandwidth usage and load time and
the images also first jump around while rearranging.

Ideally the JavaScript for masonry determines the right/roughly right size and
provides it in sizes-attribute so the browser can pick the right image file
(srcset already provided by WordPress).

[matthiaswagnerwy]

maybe you could provide a quick fix with a filter for the image size in the template file? so we could at least define the size in our own functions.php?