documentclass{article}
usepackage[all,cmtip]{xy}

xymatrixrowsep{20mm}xymatrixcolsep{20mm}

begin{document}

[
xymatrix{ A ar[r] ar[d] & B ar[d] \ C ar[r] & D }
]

[
xymatrixrowsep{20mm}xymatrixcolsep{20mm}
xymatrix{ A ar[r] ar[d] & B ar[d] \ C ar[r] & D }
]

[
xymatrixrowsep{30mm}xymatrixcolsep{30mm}
xymatrix{ A ar[r] ar[d] & B ar[d] \ C ar[r] & D }
]

[
xymatrix@R=30mm@C=30mm{ A ar[r] ar[d] & B ar[d] \ C ar[r] & D }
]

end{document}