The marble sarcophagus was initially found to be too short for the body of Willibrord 1 but subsequently proved to be longer than it.
Primary Source Info
... sarcofago marmoreo ...
Persons associated with this Event:
(Brethren at Echternach) : The marble sarcophagus at first was found to be half a foot too short for the body of the servant of God [sc. Willibrord 1] and the brothers (Anonymi 1180) were very upset at this and at a loss as to what they should do. They often discussed where they might find a coffin suitable for the holy body. Suddenly it was found that the sarcophagus was longer than the whole body of the man of God by as much as it had previously before been shorter.
(Missionary; archbishop of the Frisians, 695-739; abbot of Echternach, 697/698-739)