Consider the following MWE:
\documentclass{article}
\usepackage[all]{xy}
\begin{document}
\begin{equation}
\xymatrix{
\bullet \ar[r]\ar[d] & \bullet\\
\bullet
}
\end{equation}
\end{document}
I know that I can adjust row and column dimension with @R or @C. My question is: How can I set the row dimension equal to half the default column dimension? I.e. without affecting the column dimension?

@R=4pc @C=2pc– egreg Dec 02 '17 at 22:14