How can I give the Intellij compiler more heap space?

Current version:

Settings (Preferences on Mac) | Build, Execution, Deployment | Compiler | Build process heap size.

Older versions:

Settings (Preferences on Mac) | Compiler | Java Compiler | Maximum heap size.

Compiler runs in a separate JVM by default so IDEA heap settings that you set in idea.vmoptions have no effect on the compiler.

Since IntelliJ 2016, the location is File | Settings | Build, Execution, Deployment | Compiler | Build process heap size.