Here is a DQL statement to find the total document size by cabinet. You may be aware that dm_document has two different attributes for content size:
The first one, r_content_size, can only store sizes upto 2 GB. r_full_content_size stores the correct size if the content is larger than 2 GB. Weird. I know.
select cab.object_name, sum(doc.r_full_content_size) from dm_document doc, dm_cabinet cab where doc.i_cabinet_id = cab.r_object_id group by cab.object_name