Say below simple example:
\documentclass[border=1mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,arrows}
\begin{document}
\begin{tikzpicture}
\draw [-{Stealth[length=3mm, width=2mm]}] (0,0.5) -- (3mm,0.5);
\end{tikzpicture}
\end{document}
What's the line width (default value is 1) in mm should be?