By SIZE do you mean the number of VMs inside the template?
You can get all organization templates with the vAppTemplate query.
GET api/query?type=vAppTemplate&fields=name,numberOfVMs
'fields' says that the result will contain only the 'name' and 'numberOfVMs' fields.
You can also apply a filter:
GET api/query?type=vAppTemplate&fields=name,numberOfVMs&filter=name==un*
The request above will return all vAppTemplates starting with 'un'.
Check Query Service documentation for more information.
vCD 1.5 API Guide: http://www.vmware.com/pdf/vcd_15_api_guide.pdf
vCD 5.1 API Guide: http://pubs.vmware.com/vcd-51/topic/com.vmware.ICbase/PDF/vcd_51_api_guide.pdf