The next code produce a a6 format paper with pdflatex
that fit well, but with lualatex
the layout is a6 on a a4 paper sheet.
Passing a6paper
option to geometry package doesn't solve the problem.
How have a correct papersize.
\documentclass[a6paper]{article}
\usepackage[margin=1.5cm]{geometry}
\begin{document}
Hello word !
\end{document}