As far as size, for paperbacks or hardcovers, you can cut it into two pages, Left and Right, so the whole thing will fit. Ebooks don't really matter because kindle readers, phones, apps, etc, can focus in on the image.
When I did my family tree for my Azure series, I focused more on appearance than content. In other words, I was looking more for it looking pretty than its actual content. If you click on my newest cover under here in my signature, Princess Sojourn, it'll take you to Amazon where you can look at page four in Amazon's sample. There's my family tree for the royal family in my epic fantasy series.
This is supplement content so I would focus more on appearance than content. I think the appearance is super important, if you're gonna do it.