The max size for Windows Azure drive is 1T
M$ only charged for the data in it, not for the size
My question is: why not just create an Azure Drive at size 1T
The reason is pretty simple if you tried it. Namely, while you are not charged except for the data inside the drive, it does count against your quota limit. So, if every drive was 1TB, then you could create only 99 drives (think overhead here) before your storage account quota was gone. Also, yes, it does take longer to create a 1TB drive versus a smaller one (in practice).