Glad it worked :)

I'll try to fill the gap, so maybe things will make more sense.

The API you dealt with has 2 basic services - Disk and Disk_Attachment.

In 4.3, once you're interested in a disk attached to a VM, the service you need to use is disk_attachment.

(So the GET, PUT POST refers to the disk_attachment service).

Basically, in order to edit a disk (any disk), you need to attach it to a VM (any VM), and only then update its parameters (name, interface, size, etc).

Even if you want to edit a floating disk (a disk you've just created and wants to change its name for instance), you need to attach it to a VM, edit its name and then detach it from the VM back, so it will be floating again.

In 4.4, we've changed that behavior, so editing disks will be available also for floating disks

That means that you can edit the disk "as disk", not necessarily attached to a VM, and use the disk service.

So in your case, the service you need to use in order to update is disk_attachment.

Now, regarding the provisioned_size field, on your GET request, you've received the following response:

<disk_attachment href="/ovirt-engine/api/vms/678faf6f-fa5f-4785-a365-e1b85925575f/diskattachments/f770c0d1-dd0d-40a8-a69a-a63d8db5c2cc" id="f770c0d1-dd0d-40a8-a69a-a63d8db5c2cc">
<active>true</active>
<bootable>true</bootable>
<interface>virtio_scsi</interface>
<pass_discard>false</pass_discard>
<read_only>false</read_only>
<uses_scsi_reservation>false</uses_scsi_reservation>
<disk href="/ovirt-engine/api/disks/f770c0d1-dd0d-40a8-a69a-a63d8db5c2cc" id="f770c0d1-dd0d-40a8-a69a-a63d8db5c2cc"/>
<vm href="/ovirt-engine/api/vms/678faf6f-fa5f-4785-a365-e1b85925575f" id="678faf6f-fa5f-4785-a365-e1b85925575f"/>
</disk_attachment>

This means that the disk_attachment, the disk that attached to your VM, also has a reference to the disk itself "as a disk".

By sending a GET request for /ovirt-engine/api/disks/f770c0d1-dd0d-40a8-a69a-a63d8db5c2cc, you'll get a response of the following form:
<disk href="/ovirt-engine/api/disks/f770c0d1-dd0d-40a8-a69a-a63d8db5c2cc" id="f770c0d1-dd0d-40a8-a69a-a63d8db5c2cc">
    <actions>
...
    </actions>
    <name>your-disk-name</name>
    <description></description>
    <vms>
        <vm href="/ovirt-engine/api/vms/678faf6f-fa5f-4785-a365-e1b85925575f" id="678faf6f-fa5f-4785-a365-e1b85925575f"/>
    </vms>
    <provisioned_size>2147483648</provisioned_size>
    <shareable>false</shareable>
    <status>ok</status>
    <storage_domains>
...
    </storage_domains>
</disk>

Here, you can see the disk's parameters.

So actually, by sending an update request of the form:
<disk_attachment>
  <disk>
    <provisioned_size>2147483648</provisioned_size>
  </disk>
</disk_attachment>

Means something like "go to the disk attachment of my VM, and update the disk elements with the following fields and values".

Hope it makes more sense.