According to apple documentation, it supports two payload sizes that are 2046 KB and 4096 KB. And support depends on API version. Apple has two set of API one is old API and its support is 2046 KB. Another API supports Http/2 and its max payload support is 4096 KB.
Apple Link:
https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/TheNotificationPayload.html#//apple_ref/doc/uid/TP40008194-CH107-SW7
I want to know, what is maximum payload size for apple device PUSH, if I am using Azure Notification Hub.