[CaptainLexington]

The current implementation matches the cover image to the height or the width of the last page of the PDF, whichever is greater, and preserves the aspect ratio of the original file, which results in a PDF whose first page may be a different aspect ratio from subsequent pages.

I think this is preferable to either distorting or padding the image to match the aspect ratio of subsequent pages.* It is important both that the thumbnails of the books look attractive on all reader software that supports such thumbnails, and that the reading experience not be made inconvenient.

*(I also found that sometimes using add_image_page the inserted cover would be larger than the subsequent pages, even if it matched the aspect ratio. I believe this happened when the source image was already higher resolution than the images of the pages, but I didn't do enough testing to confirm this.)

But this is just my opinion, and if as seems true this plugin already has active users, their opinions surely count as well, especially because my setup (KOReader on a BOOX Palma) is going to be pretty niche. I would love to hear what the experience on Kindle, Kobo, and Nook is like, as well as for users who read PDFs on their computers.

It seems that configuration options are inevitable, but this is something I may not be able to get around to implementing for a while (though I will gratefully review pull requests!). Thank you, Kovid, for your assistance, and obviously for making Calibre in the first place!