Approximately 400 acres

Photo from Wikimedia Commons
Great Fire of London (1666)
The area devastated by four days of uncontrolled fire/Historical
The Great Fire of London burned from September 2-6, 1666, starting at a bakery on Pudding Lane. The fire destroyed about 13,200 houses, 87 churches, and most of the buildings of the City of London within the old Roman walls. The burned area covered approximately 1.6 square kilometers (400 acres), roughly 80% of the City of London. Despite the scale of destruction, only six verified deaths were recorded, though the actual number was likely much higher. The fire led to Christopher Wren's rebuilding of St. Paul's Cathedral and 51 other churches.
Measurements
From Tower to Temple
Thames to city walls
Old Roman wall circuit
Hot enough to melt pottery and iron locks