[Xy-pic] large arrow tips?

[Peter Selinger]

Hi,

I would appreciate some help with the following problem. 

When using Xy-pic with SliTeX, the various arrow heads don't scale
with the size of the font of the rest of the document. I tried using
the \SelectTips command to change their size, but this has no visible
effect.

How can I get larger arrow heads? The below file is a simple example.
On a slide show on an LCD projector, the arrow heads will be virtually
invisible to the audience, and in particular, the difference between A
and B will not be visible from the back of the room.

Note that the file below also includes the example from the Xy-pic
reference manual, section 10. However, for "family=cm" and
"family=eu", I can only change the "size" to 10,11,12, with the
changes in the output being almost imperceptible. Any larger size
leads to font errors. For "family=xy", the size is apparently ignored
entirely, as I don't get errors, and don't see any change in the
output.

Is there any way I can, say, double the size of the arrows? 

Thanks, -- Peter


----------------------------------------------------------------------

\documentclass{slides}
\usepackage[all]{xy}
\xyoption{ps}
\xyoption{dvips}

\CompileMatrices
\SelectTips{xy}{30} % the second argument is useless
\UseTips

\begin{document}

\begin{slide}
  \[
  \let\labelstyle\objectstyle
  \vcenter{\xymatrix{
      A \ar@{-}[rr]^<>(.5){\rm id}
      \ar@{}[rr]|<>(.5)/.3ex/{\dir2{>}}&&A
      \\
      B \ar@{-}[rr]^<>(.5){\rm id}
      \ar@{}[rr]|<>(.5)/-.3ex/{\dir2{<}}&&B
      \\
      C \ar[r]^{f} & C
      }}
  \]
  \[
  \SelectTips{cm}{10}
  \xy*{} \ar
   @{*{\UseTips\dir_{<<}}-*{\UseTips\dir{>}}}
   (20,5)*{} \endxy
  \]

\end{slide}

\end{document}