People read Kindle books on phones, computers and ereaders - all different size screens.
People using Kindle readers can change the font, the font size, the margins and the line spacing.
If the preview viewer offers you different screen sizes, try them. if the preview viewer offers different fonts, font sizes, margins and line spacing, try them.
Don't worry too much how it looks with very large fonts - people who use those have bad eyesight and need the large font to be able to read.