Size limits or guarantees

Signed-off-by: Christoph Neijenhuis <christoph.neijenhuis@commercetools.de>

spec.md

@@ -15,6 +15,7 @@ This document is a working draft.
 - [Type System](#type-system)
 - [Context Attributes](#context-attributes)
 - [Data Attribute](#data-attribute)
+- [Size Limits](#size-limits)
 - [Example](#example)
 
 ## Overview
@@ -289,6 +290,32 @@ encapsulated within the `data` attribute.
 * Constraints:
   * OPTIONAL
 
+# Size Limits
+
+In order to increase interoperability, the following size limits apply to any
+CloudEvent:
+
+* The CloudEvent serialized as JSON (minified, i.e. without white-space) MUST
+  NOT exceed a size of 128KB.
+* The CloudEvent MUST NOT have more than 100 top-level attributes.
+
+CloudEvent producers SHOULD only create CloudEvents within these limits.
+CloudEvent consumers MUST accept all CloudEvents within these limits and SHOULD
+reject messages that violate these limits.
+
+# Size Guarantees // Another option
+
+In order to increase interoperability, all CloudEvent consumers MUST accept
+messages up to the following sizes:
+
+* The CloudEvent serialized as JSON (minified, i.e. without white-space) does
+  not exceed a size of 128KB.
+* The CloudEvent does not have more than 100 top-level attributes.
+
+It is RECOMMENDED for CloudEvent producers to only create CloudEvents within
+these limits, unless they can be sure all consumers support larger sizes.
+CloudEvent consumers MAY reject messages that violate these limits.
+
 # Example
 
 The following example shows a CloudEvent serialized as JSON: