\documentclass{article}
\usepackage{tabularray}
\begin{document}
\begin{tblr}{
hlines,
vlines,
rows={15mm},
columns={15mm},
}
a & bbb bbb bbb bbb bbb bbb bbb bbb bbb bbb bbb bbb bbb bbb \\
c & d \\
\end{tblr}
\end{document}
As you can see, the cells are definitely not 15 mm x 15 mm. What are my options to force those cell dimensions?

tabularrayexpect, that cell content should not protrude bottom of cell, so it extend cell high accordingly. So far, as I know, you are the first, who require that text must protrude bottom of cells ... – Zarko Jun 26 '22 at 00:59tabularray, see this answer: https://tex.stackexchange.com/a/648978/106776 – L.J.R. Jun 26 '22 at 08:12