When you want to grow your current Big Storage partition on Windows Server you can login to the control panel and go to BladeVPS > Big Storage. Here you can expand your Big Storage by using the plus-button. The purchase can be completed by selecting Order Big Storage.
Windows Server will automatically add your newly ordered space. Through Server Manager in your terminal you can choose Tools > Computer Management. You will see an Unallocated Volume behind your current disk when you go to Storage > Disk Management. To grow the current disk with this Unallocated Volume you can right-click your current volume and select Extend Volume. A Setup Wizard will open in which the unallocated volume is set up correctly by default. You only have to select Next and Finish.
After completing the Wizard the extra space of your Big Storage should be fully available.