It may not help you but I have tried it on a few vApps and it returns their disk space in MB.
Nt sure why you do not get it.
Have you tried on different ones ?
System.log your vApp.toXML();
In mine the size seems to come from this section:
<rasd:ElementName>Hard disk 1</rasd:ElementName>
<rasd:HostResource vcloud:capacity="8192" vcloud:busSubType="lsilogic" vcloud:busType="6"/>
here is the same section I recieve back with vApp.toXml
I need to get the attention from someone who has access to the back-end source to understand what is going on. You can also resort to GSS to make this an official request.
This property is provided by the vCD Java SDK.
What it returns is the sum of all VM hard disk sizes within the vApp.
If the vApp has at least one VM that has a hard disk I see no reason for this to fail.