The Dutch artificial island 'Flevopolder' is ~970 square km. So, I wouldn't say giant when there is an artificial island that has a 373x larger surface area.
But that’s a polder made mostly by building dykes and draining the land rather than raising up the ground level across 970 square km. Flevopolder is about 4m below sea level.
I think the plan here is different and will not be done by building up dykes and draining the area inside.
These kind of numbers are not intuitive. Don't forget that you are working in volume of material. Making something only 1m high across 1 sq mile is around 2.5 million cubic metres.
Sure it’s not the size of Japan or something, but it’s not tiny.