Yet this canard persists on page 21 of Engaging Our Theological Diversity:
Universalism was a populist movement that ranked among the largest six or seven denominational groups in America at its height in the mid-nineteenth century.
This claim seem wholy unmerited, however often it is repeated. Note there's no footnote. A review of mid-nineteenth-century Universalist Registers would also support a small denomination than number six or seven.