Here is a snippet from my WDSUnnattend.XML file. It wipes the disk and creates a single partition to fill the disk, then installs to that partition.
<ImageInstall>
<OSImage>
<InstallTo>
<DiskID>0</DiskID>
<PartitionID>1</PartitionID>
</InstallTo>
<WillShowUI>Never</WillShowUI>
</OSImage>
</ImageInstall>
<DiskConfiguration>
<Disk wcm:action="add">
<DiskID>0</DiskID>
<WillWipeDisk>true</WillWipeDisk>
<CreatePartitions>
<CreatePartition wcm:action="add">
<Type>Primary</Type>
<Order>1</Order>
<Extend>true</Extend>
</CreatePartition>
</CreatePartitions>
<ModifyPartitions>
<ModifyPartition wcm:action="add">
<Active>true</Active>
<Extend>false</Extend>
<Format>NTFS</Format>
<Label>System</Label>
<Letter>C</Letter>
<Order>1</Order>
<PartitionID>1</PartitionID>
</ModifyPartition>
</ModifyPartitions>
</Disk>
<WillShowUI>OnError</WillShowUI>
</DiskConfiguration>