diff --git a/event-schemas/schema/v1/core-event-schema/state_event.json b/event-schemas/schema/v1/core-event-schema/state_event.json
index f70118f4..88b4900a 100644
--- a/event-schemas/schema/v1/core-event-schema/state_event.json
+++ b/event-schemas/schema/v1/core-event-schema/state_event.json
@@ -8,7 +8,7 @@
   "properties": {
     "state_key": {
       "type": "string",
-      "description": "A unique key which defines the overwriting semantics for this piece of room state. This value is often a zero-length string. The presence of this key makes this event a State Event."
+      "description": "A unique key which defines the overwriting semantics for this piece of room state. This value is often a zero-length string. The presence of this key makes this event a State Event. The key MUST NOT start with '_'."
     },
     "prev_content": {
       "type": "object",
diff --git a/event-schemas/schema/v1/m.room.name b/event-schemas/schema/v1/m.room.name
index b43f02cc..0e0b25be 100644
--- a/event-schemas/schema/v1/m.room.name
+++ b/event-schemas/schema/v1/m.room.name
@@ -11,7 +11,7 @@
             "properties": {
                 "name": {
                     "type": "string",
-                    "description": "The name of the room."
+                    "description": "The name of the room. This MUST NOT exceed 255 bytes."
                 }
             },
             "required": ["name"]
diff --git a/specification/0-events.rst b/specification/0-events.rst
index 16948462..ae5a698a 100644
--- a/specification/0-events.rst
+++ b/specification/0-events.rst
@@ -12,6 +12,23 @@ server-server and application-service APIs, and are described below.
 {{common_state_event_fields}}
 
 
+Size limits
+-----------
+
+The total size of any event MUST NOT exceed 65 KB. There are additional
+restrictions on sizes per key:
+
+ - ``user_id`` MUST NOT exceed 255 bytes (including domain).
+ - ``room_id`` MUST NOT exceed 255 bytes.
+ - ``state_key`` MUST NOT exceed 255 bytes.
+ - ``type`` MUST NOT exceed 255 bytes.
+ - ``event_id`` MUST NOT exceed 255 bytes.
+ - ``user_id`` MUST NOT exceed 255 bytes.
+
+Some event types have additional sizes restrictions which are specified in
+the description of the event. Additional keys have no limit other than that
+implied by the total 65 KB limit on events.
+
 Room Events
 -----------
 .. NOTE::
diff --git a/specification/0-intro.rst b/specification/0-intro.rst
index 445b32ef..e5cf1ac8 100644
--- a/specification/0-intro.rst
+++ b/specification/0-intro.rst
@@ -296,7 +296,7 @@ be obtained by visiting the domain specified. They are case-insensitive. Note
 that the mapping from a room alias to a room ID is not fixed, and may change
 over time to point to a different room ID. For this reason, Clients SHOULD
 resolve the room alias to a room ID once and then use that ID on subsequent
-requests.
+requests. Room aliases MUST NOT exceed 255 bytes.
 
 When resolving a room alias the server will also respond with a list of servers
 that are in the room that can be used to join via.