Set a 'max_stanza_size' for incoming s2s listener

Specify a 'max_stanza_size' limit for incoming s2s connections in the
example configuration, but use a relatively large value in order to
minimize the risk of this limit being hit by legitimate traffic.

ejabberd.yml.example

@@ -51,6 +51,7 @@ listen:
     port: 5269
     ip: "::"
     module: ejabberd_s2s_in
+    max_stanza_size: 524288
   -
     port: 5443
     ip: "::"