To cite OTL examples, its got to be something to do with the Dutch. The Japanese built an artificial island in Nagasaki harbor in the 17th C. for the Dutch (actually for the Portuguese but they had been kicked out) for trading purposes (they weren't allowed to go to the mainland)--Deijima. It seems to have been earth and rock fill and was almost a hectare in size.
Some of the polders the Dutch built back home, were strictly speaking, artificial islands. Some of the early ones fit your requirement. I don't know what realistic sizes were attempted in the early modern period.
Concrete doesn't seem to be a prerequisite for early artificial island construction. Earth and/or rock were the preferred materials. Not including Aztec floating islands, of course.
The Venetians enlarged and consolidated the marshy islands in their lagoon by driving 1000s of wooden piles into the lagoon bottom over the centuries. In a sense, Venice is a collection of artificial islands.