Hi,
You will be charged for full size in all three cases (Standard SSD, Premium SSD, Standard HDD) that you mentioned. For example, for 128 GiB LRS OS Disk in East US region:
Premium SSD P10 $19.71/month + outbound data transfers (if any)
Standard SSD E10 $9.60/month + transaction charges + outbound data transfers (if any)
Standard HDD S10 $5.89/month + transaction charges + outbound data transfers (if any)
Please see Managed Disks pricing using link below:
https://azure.microsoft.com/en-us/pricing/details/managed-disks/
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP