Apple Push Notification service (APNs) is the centerpiece of the remote notifications feature. Each push notification includes a payload. Prior to iOS 8, the maximum notification payload size is 256 bytes. Apple Push Notification service(APNs) refuses any notification that exceeds this limit.
In iOS 8 and later, the maximum size allowed for a notification payload is increased to 2KB (i.e 2048 bytes)
Now, HTTP/2 APNs is officially supported. (developer.apple.com/news/?id=12172015b)
Hence, The maximum size allowed for a notification payload depends on which provider API you employ. When using the HTTP/2 provider API, maximum payload size is 4KB (i.e. 4096 bytes), and this limit apply to all versions of iOS and OS X. Using the legacy binary interface, maximum payload size is 2048 bytes.