You can check the amount that it is being transferred during the update of the snapmirror by running the command below:
snapmirorr show -destination-path <vserver:volume> -instance
Source Path: vserver1:vol1
Destination Path: vserver2:vol1_dest
Total Progress: -- Total amount of data transferred for the current transfer operation
Last Transfer Size: -- Total amount of data transferred during the previous transfer operation if it was successful.
Total Transfer Bytes:-- Cumulative bytes transferred for the relationship.
If you are looking for the amount data that is going to be transferred during a snapmirror; it is not something that you will be able to show as the snapmirror is taken using a snapshot that is created during the snapmirror operation to replicate to the destination.