TikZで矢印の長さを表す補助線を引く例

\documentclass[dvipdfmx]{jsarticle}
\usepackage{tikz}
\pagestyle{empty}
\begin{document}

\begin{tikzpicture}

  \tikzset{VECTOR/.style={color=black,thick,->,>=stealth}}
  \tikzset{BENDTO/.style={color=black!50!white,very thin,looseness=0.5,bend left=60}}

  \coordinate (O) at (0,0);
  \coordinate (P) at (30:3);

  \draw[BENDTO] (O) to (P);
  \draw[VECTOR] (O) -- (P);

\end{tikzpicture}

\end{document}