how to define disk size with clone vm

Any one know how to define disk size when clone one vm ?

following code doesn't work :

 my $disk = VirtualDisk->new (capacityInKB => $size);

 my $devspec = VirtualDeviceConfigSpec->new(operation => VirtualDeviceConfigSpecOperation->new('add'),         device => $disk,        );



 my $cust_hardware = VirtualMachineConfigSpec->new (         deviceChange => [$devspec] ,



 );

 my $clone_spec = VirtualMachineCloneSpec->new( 



 config => $cust_hardware, 

 );