Napkins are usually square. The following are the sizes used for different occasions:
Formal, Multiple Course Meals
Large (22 to 26 in square)
Buffet Service
Medium to large (18 to 24 in square or 12 × 22 in)
Informal Dinners
Medium (18 to 20 in square)
Luncheons
Smaller (14 to 16 in square)
Cocktails
Very small (9 in square, 4 × 6 in, or 6 × 8 in)