Hi,
This can be done in SAP Customization / SPRO
Pl. check :
Cross-Application Components>Document Management>Control Data>Define Document types
Here the size limit can be defined.
Also check : http://scn.sap.com/community/plm/document-management/blog/2014/06/20/how-to-create-dir-and-document-type-in-dms
Hope this helps
Regards,
Remi