Is it possible to modify a volume size ?

While you cannot hot modify a BSU volume size, there are two ways to cold modify a BSU volume size:

1. To increase or decrease the size of a volume, create a second BSU volume, and attach it to the VM. Then, copy all data from the first volume, detach it, detach the first volume, and finally replace it with the new one.
2. To only increase the size of a volume, make a snapshot of the first volume and create a volume from this snapshot with more space. Then, attach the new volume instead of the original and extend the filesystem (FS). 

You can also use several aggregated volumes using LVM on Linux or natively on Windows, or use a specific filesystem like BTRFS. For more information, see Improving a Volume Input and Output Performances with LVM.