How to change MaxHeap for Ant scripts?

One of my Ant scripts runs on an OutOfMemoryError.

I was trying to increase Ants heap size by providing a JVM setting.

In the settings dialogue for the Ant script there is a comment that I have to use the -J prefix for JVM parameters.

So I tried:
-JXmx1024m

and
-J -Xmx1024m

but in both cases the Ant script does not run anymore and IDEA only says "Ant script runs successfully in 0 sec".

Is there any magic to get this running?

Thanks in advance!