Example file: https://en.wikisource.org/wiki/Explorers_of_the_Dawn_(February_1922)

This book uses https://en.wikisource.org/wiki/File:Explorers_of_the_Dawn_cover.png, which is about 12MB, at the full resolution (2,205 × 3,504px)

This bumps the export filesize to be really large, for no real benefit. The cover size doesn't need to be that big - usually it's only used for tile views on an e-reader, so something mild like 400-600px will probably do.