How to prevent filesystem resize of guest boot device on first boot ?

Issue

• Is there a way to partition the root disk when initially booting an instance ?

• For example, when flavor (disk: 80GB) is used, 80GB is allocated to /dev/sda first partition

• Scenario 1: Can we allocate space to partitions as follows instead ?

/dev/sda - 40GB
/dev/sda1 - 20GB
/dev/sda2 - 20GB

• Scenario 2: Can we keep the unused space as unallocated :

/dev/sda - 40GB
Unallocated - 40GB