Running out of heap space using Maven

I encountered the following error when running mvn site

[INFO] ------------------------------------------------------------------------
[ERROR] FATAL ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Java heap space
[INFO] ------------------------------------------------------------------------
[INFO] Trace
java.lang.OutOfMemoryError: Java heap space

So before running mvn site command I set MAVEN_OPTS=-Xmx512m

That fixed it, all is well.