Checked the source under this one. Cliopatria (Seshat Global History Databank) spans 3400 BCE to 2024 CE: about 1,600 polities across ~14,000 records, published in Nature Scientific Data 2025, CC BY. Its polygons store area in an equal-area projection (EPSG:6933), so your km2 comparison holds across latitude. One small note: the dataset's unit is the polity, not the empire, so the 100 is your cut.