how to resize easily a pstricks picture?

Changing the units (unit itself and linewidth) does resize the image:

\documentclass{article}

\usepackage[pdf]{pstricks}
\newpsobject{pslineA}{psline}{linewidth=6\pslinewidth, linecolor=green}
\newpsobject{pslineB}{psline}{linewidth=10\pslinewidth, linecolor=red}

\begin{document}

\begin{pspicture}(5,5)
  \psline(0,1)(5,1)
  \psline[linewidth=1mm](0,2)(5,2)
  \pslineA(0,3)(5,3)
  \pslineB(0,4)(5,4)
\end{pspicture}\\[2cm]

\psset{unit =1.5cm, linewidth=1.5\pslinewidth}
\begin{pspicture}(5,5)
  \psline(0,1)(5,1)
  \psline[linewidth=1mm](0,2)(5,2)
  \pslineA(0,3)(5,3)
  \pslineB(0,4)(5,4)
\end{pspicture}
\end{document} 

setting the linewidth as a multiple of \pslinewidth sets it as an absolute value, the reason why the \psset{unit=1.5} doesn't change the setting. Compare it with

\newpsobject{pslineA}{psline}{linewidth=0.05, linecolor=green}
\newpsobject{pslineB}{psline}{linewidth=0.2, linecolor=red}

which is a relative linewidth setting which refers to the current units, e.g. 1cm. Then you can say \psset{unit=..} or

\psscalebox{<value>}{%
  \begin{pspicture}(..)(..)
  ...
  \end{pspicture}}

for rescaling the pspicture environment.