Label above node in TikZ

Actually, tikz nodes have text inside and labels outside:

\documentclass{standalone}
\usepackage{tikz}
\begin{document}

\begin{tikzpicture}[scale=0.8,every node/.style={draw=black,circle}]
    \node[label={\small 1/16}] (a) at (0,0) {a};
    \node (b) at (2,0) {b};
    \draw[->] (a) to (b);
\end{tikzpicture}

\end{document}

A PSTricks solution using the pst-node (for the drawing itself) and xfp packages (for the floating point calculations):

\documentclass{article}

\usepackage{pst-node}
\usepackage{xfp}

\def\labelSep{\fpeval{28.75*\radius+3.375}} % found by experimenting

% parameters
\def\arrowLength{1.8}
\def\radius{0.3}

\begin{document}

\begin{pspicture}(\fpeval{4*\radius+\arrowLength},\fpeval{2*\radius+0.35})
% change `0.35` in the calculation of the height of the bounding box
% according to the contents on top of the node
  \cnode(\radius,\radius){\radius}{A}
  \rput(A){a}
  \cnode(\fpeval{3*\radius+\arrowLength},\radius){\radius}{B}
  \rput(B){b}
  \ncline{->}{A}{B}
  \uput{\labelSep pt}[90](A){\small\texttt{1/16}}
\end{pspicture}

\end{document}

Notice that all you have to do is choose the values of the parameters and the drawing (including the bounding box) will be adjusted accordingly.