How do I resize the margins

[jonathonLee]

The outside margins include the top, bottom, and outside edges of your book. Most books print best with at least 0.5" (12.7 mm) outside margins.

If you need to set a narrower outside margin, do not go below 0.375" (9.6 mm) wide for books with bleed or 0.25" (6.4 mm) wide for books without bleed. Outside margins that do not meet the minimum requirement will result in cut-off text and will fail in our Online Previewer.

[mikekaganski]

I suppose this is a practical advise based on personal experience, not a question to be answered?

Also, which our Online Previewer are you talking about?

[JohnHa]

“How do I resize the margins”

Format > Page > Page …, and set them to whatever value you want.