@Michael Shparber Thank you for your query!!!
Unmanaged disk has a fixed price per disk size (Per month), whatever the disk usage is.
Option to pay hourly for the disk is available for managed disks in Azure VM. Azure Managed Disks are t recommended disk storage offering for use with Azure virtual machines for persistent storage of data. You can use multiple Managed Disks with each virtual machine. We offer four types of Managed Disks — Ultra Disk, Premium SSD Managed Disks, Standard SSD Managed Disks, Standard HDD Managed Disks. For Azure Managed Disks, you will be billed on an hourly basis.
You can read more about it here.
You can convert your unmanaged disk to managed one using this doc or this based on type of VM which has step by step guide to do same.
You won't be able to reduce disk size since it since new size should be greater than the existing disk size as mentioned here as well.
Hope it helps!!!
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