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Unknown page size

papersize names a page size Keystone does not offer. The build stops rather than printing at some other size.

What it looks like

papersize: a4paper

ERROR: papersize 'a4paper' is not a page size Keystone names
  Valid: a0, a1, a2, a3, a4, a5, a6, b0, b1, b2, b3, b4, b5, b6, c0, c1, c2, c3,
         c4, c5, c6, ansia, ansib, ansic, ansid, ansie, letter, legal, executive
  For any other trim, give the dimensions to geometry instead, e.g. geometry:
  paperwidth=5in,paperheight=5in
  See https://keystone.knight-owl.dev/errors/unknown-papersize/

What it means

Page sizes here are named without the paper suffix — a4, not a4paper. The suffixed spelling is real, but it belongs to geometry, which names sizes of its own; papersize takes the bare name.

Keystone checks that name because nothing downstream does: a size LaTeX does not recognize is dropped without a word, and the book prints at some other size with nothing in the PDF to say so.

Take a name the message lists, or size it in geometry

Page sizes carries the list with the dimensions each name stands for. A trim with no name goes to geometry as dimensions instead, with papersize left empty.

Most print-on-demand formats, square children's books, anything cut to order:

papersize:
geometry: paperwidth=6in,paperheight=9in,margin=0.75in
  • Page sized twice — papersize and geometry both answering for the page.
  • Unsupported font size — the same shape for the base font size, which falls back with a warning rather than stopping.