Its not the earliest cross discovered, but its discovery reminds us that only some 100 years after Constantine legalised the early church, crosses, as church decoration were becoming common.
Quote: "Bulgarian archaeologists have discovered a large bronze cross during excavations in an Early Christian basilica in the ancient city of Parthicopolis, which is located in today’s southwestern town ofSandanski.
The find is unique because the cross is approximately 1500-years-old – it is dated back to the 5th-6th century AD – but also because of its size, reports Bulgarian TV channel News7.
At 55 cm in length the Early Christian bronze cross discovered in one of the four ancient basilicas inSandanski known as the Bishop Basilica is the largest of its kind to have ever been dug up in Bulgaria."
More information at: http://archaeologyinbulgaria.com/2015/03/02/bulgarian-archaeologists-find-5th-century-bronze-cross-in-early-christian-basilica/