Retain 640 character limit

app/serializers/rest/instance_serializer.rb

@@ -11,7 +11,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
 
   attributes :domain, :title, :version, :source_url, :description,
              :usage, :thumbnail, :languages, :configuration,
-             :registrations
+             :registrations, :max_post_chars
 
   has_one :contact, serializer: ContactSerializer
   has_many :rules, serializer: REST::RuleSerializer
@@ -89,6 +89,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     }
   end
 
+  def max_post_chars
+    640
+  end
+
   private
 
   def registrations_enabled?