[parkher]

Aura does not support landscape for books, only for pdf.
Actually, Aura would be a bit of an overkill for landscape if it supported it.
I think that the text should be about 8.8 cm wide as in printed paperbacks. Provided you are using a similar size font as in printed paperbacks. Then on Aura you still have some margins left. More would be better, but enough. I got Aura specifically so I don't have to use landscape. But without landscape it is difficult to hold the reader for a longer time, no way to hang it on a finger.

If you have a 6" reader, try landscape on it.
6" in landscape orientation becomes 12cm wide, so you can have:

1 cm margin + 10 cm text + 1 cm margin
1.5 cm margin + 9 cm text + 1.5 cm margin
etc.
See which feels better for you - it depends on the font size, if you are using smaller font you might want to have wider margins and narrower text, if larger font - wider text.

The width of text should depend on the size of letters. But nobody can agree what is the best ratio: 1.5 alphabets - 1.75 alphabets - 2 alphabets - 2.25 alphabets - 2.5 alphabets - even 2.75 alhpabets are mentioned.
In mass market paperbacks the requirement seems to be 40-60 letters per full line of text. Correspondingly, there is some range of font sizes in paperbacks too.