The Exchange Storage report can be used to determine the scoping size of the source tenant prior to a migration.
The table shows, for every user in the tenant, core storage details about the mailbox including size, last login, how many items and if there is an archive the size and number of items in the Archive.
By knowing the size of the mailboxes you can prepare migration waves that align with your schedule. Larger mailboxes will take longer than smaller mailboxes, for example, so a much larger list of smaller mailboxes could be migrated as required. For more information about Cloud Commander, go here.
As with all of the reports, this information can be customized, filtered, and scheduled.