Azure blob storage is billed based to how much data you use. So you can have an 1023 GB disk but if you use only 20 GB you will be billed for 20 GB. But, ???? , if you write more data, lets say 50 GB and then you erase them, the free space will not automatically be released.
sandrinodimattia, https://github.com/sandrinodimattia, released an app that allows to check the actual size of a VHD on Azure. It works on both ASM and ARM.
You can download the executable at: https://github.com/sandrinodimattia/WindowsAzure-VhdSize/releases
The command is:
wazvhdsize.exe “storageaccountname” “storageaccountaccesskey==” containername