dashed signature

There's no “official” LaTeX interface to leaders, so we need some low level programming.

\documentclass{article}

\newcommand{\signaturespace}[2]{%
  % #1 = width of the dotted line
  % #2 = legend
  \begingroup
  \renewcommand{\arraystretch}{0}%
  \begin{tabular}[t]{cc}
  \hspace*{0pt}%
  \cleaders\hbox{\kern.1pt.\kern.1pt}\hskip#1\relax
  \hspace*{0pt}%
  \\[2pt]
  \scriptsize#2
  \end{tabular}%
  \endgroup
}

\begin{document}

\signaturespace{5cm}{(czytelny podpis studenta)}

\end{document}

tikz to the rescue

\documentclass{article}


\usepackage{tikz}
\newcommand{\sig}[1]{\begin{tikzpicture} \draw[ultra thick,densely dotted] (0,0)--(4,0) (2,0) node[below]{#1};  \end{tikzpicture}}

\begin{document}
\sig{your signature}

\end{document}