As explained by both Asma Elmangoush and Rivalino Matias Jr, you cannot find a "standard" size, nor determine the average for the overall Cloud network communication.

Instead, you can select a particular application and a particular protocol (if the application uses multiple ways to communicate with different parts of the whole system) and perform measurements in your network to that particular Cloud hosted application (network listening) and after a finite about of time and a finite number of recorded or scanned/measured packet, you can perform statistical analysis and find the smallest, largest, average size of the packet communicated via the selected protocol between clients in your network and that Cloud hosted application.

This is the case for most common protocols (that there is no precise packet size). However, there are certain specialised protocols that do enforce a fixed length packet, regardless of the data that is being sent. In most cases, these packets contain data smaller than the packet, but some protocols allow for chunked (split) data being sent over multiple packets and than assembled at destination. I am not talking about network layer protocols here. I am talking about very specific application layer protocols developed and designed to function that way for one reason or the other.