If you accept the outer integument of the Palaeozoic medullosalean ovules to be homologous with a cupule (which appears to be widely accepted nowadays) then Pachytesta incrasata Brongniart ex Renault must be among the largest cupules, over 100 mm long and 50 mm wide.

Here is a medullosean ovule (morpho genus Pachytesta ) housed at Grenoble Alp university (OSUG's collection). Taxon provides from Belgium in Charleroi basin, middle Pennsylvanian age, 90 millimeters long and associated with Alethopteris foliage. Best regards.