The OneDrive Info report in the Cloud Commander Report pack outlines overall size of the OneDrive footprint for the tenant to be migrated, focused on per user statistics.
The table gives both the overall count and the size in Megabytes of the individual OneDrives by user.
By knowing the size and activity profile of the OneDrives in the source tenant you can prepare migration waves that align with your schedule. Migrate inactive OneDrives, for example, to test the migration or more smaller sets of larger OneDrives to ensure the migration wave finished within your time window. For more information about Cloud Commander, go here.
As with all of the reports, this information can be customized, filtered, and scheduled.