This chart will help you translate container sizes for standard clay pots and black nursery pots and give you an approximation of how much soil each will require (again these are dry soil measures):
4 inch pot (10 cm) = 1 pint (0.5L)
5-6 inch pot (13-15 cm) = 1 quart (1L) = 0.03 cu. ft.
7-8 inch pot (18-20 cm) = 1 gallon (4L) = 0.15 cu. ft.
8.5 inch pot (22 cm) = 2 gallon (7.5L) = 0.3 cu. ft.
10 inch pot (25 cm) = 3 gallon (11L) = 0.46 cu. ft.
12 inch pot (30 cm) = 5 gallon (19L) = 0.77 cu. ft.
14 inch pot (36 cm) = 7 gallon (26L) = 1 cu. ft.
16 inch pot (41 cm) = 10 gallon (38L) = 1.5 cu. ft.
18 inch pot (46 cm) = 15 gallon (57L) = 2.3 cu. ft.
24 inch pot (61 cm) = 25 gallon (95L) = 3.8 cu. ft.
30 inch pot (76 cm) = 30 gallon (114L) = 4.6 cu. ft.