omg, adding <phase>verify</phase>
worked for me:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-enforcer-plugin</artifactId>
<executions>
<execution>
<id>enforce-file-size</id>
<phase>verify</phase>
<goals>
<goal>enforce-once</goal>
</goals>
<configuration>
<rules>
<requireFilesSize>
<maxsize>419430400</maxsize> <!-- the final zip should not exceed 400 MB -->
<files>
<file>${project.build.outputDirectory}.zip</file>
</files>
</requireFilesSize>
</rules>
<fail>true</fail>
</configuration>
</execution>
</executions>
</plugin>