From the OED:
Queen:
A large roofing-slate, measuring approximately three feet by two feet (approx. 0.91 by 0.61 m). Now chiefly hist.
(The queen was originally the largest size in a standard system developed at the Penrhyn quarry near Bethesda, Caernarvonshire (now in Gwynedd), in the 18th cent.; its exact size varied at different periods and between different quarries).