Some basics for tablecloth sizing:
55 in. x 55 in. - Card table square (seats 4)
60 in. x 90 in. - Picnic Table (seats 6)
60 in. x 120 in. - Family Style (seats 8-10)
60 in. x 140 in. - Extra Long Family Style (seats 10-12)
90 in. Round - Fits most round tables, only the drop changes