cover size shrinks when converting/modifying epub
I am trying to replace the cover in an epub with a larger one. I downloaded a cover (1920x1400 jpg) from the publisher's website. Calibre reduced the size to 1053x1600 for display in its cover browser, which is fine. But when I updated the cover in the epub, whether by doing an epub-->epub conversion or by using the modify epub plugin, the cover actually stored in the epub gets reduced to 686x451.
I have a Kobo Glo which has screen resolution 1024x758, so I would like the cover to be about this size. How can I tell Calibre that?