Rectangular and oval tablecloths come in 52×70, 60×84, 60×104, 60×108, 60×120, 90×132, 90×156 inches; round ones in 70, 90, 96, 108, 114, 120, 132 inches; square ones in 52, 60, 70, 72, 84, 90 inches. Pick the size that is your table plus twice the drop, and go up when you fall between sizes (Sur La Table).
Outlines of the rectangular sizes drawn from the same corner, to scale. Each grid square is one foot. Sizes too close to label are in the table.