I may not be clear on what you are after, but I am going to assume you mean turning this...
BackupSizeGB=$storage.Info.Stats.BackupSize/1GB;
DataSizeGB=$storage.Info.Stats.DataSize/1GB;
… into this...
BackupSizeGB=$storage.Info.Stats.BackupSize/1TB;
DataSizeGB=$storage.Info.Stats.DataSize/1TB;
The math approach is the same, but use and if/then to determine when the GB size hits the TB threshold. For example...
if(DataSizeGB -ge 1024){DataSizeGB=$storage.Info.Stats.DataSize/1TB}
if(BackupSizeGB -ge 1024){BackupSizeGB=$storage.Info.Stats.BackupSize/1TB}
… or have two columns, on in GB and the other in TB.
See this article:
'powershellmagazine.com/2013/05/20/converting-to-size-units-kb-mbgbtb-and-pb-without-using-powershell-multipliers'