How long is the border between France and the Kingdom of the Netherlands?

About ten miles (16 kilometres), exclusively on the Caribbean island known in English as St. Martin (Saint-Martin in French and Sint Maarten in Dutch). Fairly recent changes in the status of the respective parts of this island within the government structures of their sovereign nations led them to be considered direct parts of those European nations.

The two European powers have shared this island since 1648, just after the Spanish abandoned it. Its name comes from the 11 November feast day of St. Martin of Tours, but was originally meant for the island now known as Nevis when Christopher Columbus anchored there on that day in 1493. Confused mapmakers later transferred the name to this island.

The Saint Martin-Sint Maarten border, or France-Netherlands border, is the border between the Collectivity of Saint Martin, an overseas collectivity of France, and Sint Maarten, a constituent country of the Kingdom of the Netherlands, on the island of Saint Martin in the Caribbean. The 87-square-kilometre (34 sq mi) island is divided roughly 60:40 between the French Republic (53 km2, 20 sq mi)[1] and the Kingdom of the Netherlands (34 km2, 13 sq mi)[2] by the 16 km (10 mi) border.[3] However, the two parts are roughly equal in population.

The border is completely open and can be crossed freely.

https://en.wikipedia.org/wiki/Saint_Martin%E2%80%93Sint_Maarten_border

See also https://en.wikipedia.org/wiki/Saint_Martin_(island), https://en.wikipedia.org/wiki/Sint_Maarten, https://en.wikipedia.org/wiki/Collectivity_of_Saint_Martin, https://en.wikipedia.org/wiki/Unification_of_Saint_Martin