I faced this question in preparing my recent Dragon Space: A Star Rigger Omnibus, which combined two novels originally published in paper by Tor. The second novel, Dragon Rigger, had a lovely map that had spanned two printed pages. The artist happily gave me the map, but I wasn't sure how to present it.
Eventually I decided on a 3-fold approach. I put the whole map in the ebook in landscape mode, and then the "west" and "east" halves in portrait, so the features would be a little bigger. Finally, I included a link to a
larger version of the map on my website. I don't think that many people use the web version, but it's there if they want it.