Project
Editions come from this site's cached snapshot, then anything minted since
is read from the blockchain, so the set is never short of the chain.
Output size is decided by the art script. Many derive it from the viewport,
so that setting controls them; scripts that hard-code their dimensions ignore it.
Export
What to do with the output
Unzip and upload the files to any host that serves plain HTTPS, keeping the names as they are. Then in Project Admin → Marketplace Thumbnails set the Image Base URL to that folder and the extension to match.
These images are only for marketplaces, which show a still rather than running the art script. Immutables.art itself always renders your work live from the blockchain and never uses them.