This function computes the space required
(in bytes) to clone a virtual disk, after possible format conversion.
vixError = VixDiskLib_SpaceNeededForClone(child.Handle(), VIXDISKLIB_DISK_VMFS_FLAT, &spaceReq);

Caution
VixDiskLib_SpaceNeededForClone() might not give
accurate results, or could return
VIX_E_INVALID_ARG, when used
with thin provisioned disk type
VIXDISKLIB_DISK_VMFS_THIN.