We can not increase or decrease font size for fixed layout books; we must fix the type size (font size) at the beginning itself. (micrologicglobalservices.com)
But this is not the case with fixed layout books, we must fix a paragraph to be at a certain place. (micrologicglobalservices.com)
You can see a comprehensive list of stores that accept fixed layout books at the stores page. (publishdrive.zendesk.com)