The default is 64 MB using 32-bit Java and ~83 MB using 64-bit Java. The correct sizing, when dealing with Mendix, will mostly depend on the amount of (Java) classes you use (i.e. basically the larger your userlib folder is the larger your PermGen size most likely needs to be). I think the default in our Cloud is 128MB for Java6 and 7 but since most applications are now running at least Mendix 5.16.0 and thus Java8 I couldn't quickly find it as PermGen was removed in Java8 (and replaced by Metaspace which has unlimited size by default).