Roughly 22 cm (8.65 inches) in diameter for a regulation size 5 ball. Rules state that a size 5 ball must be 68 to 70 cm in circumference. Averaging that to 69 cm and then dividing by pi gives about 22 cm for a diameter.
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The rules of the game state that the circumference should be 68 to 70 cm. (See e.g. <http://en.wikipedia.org/wiki/Football_(ball)#Dimensions>.)
Thus, the average *circumference* is probably 69 cm (about 27 1/6 in), making the diameter (69 / pi) cm, or roughly 22 cm (about 8 3/4 in).
Size 4s are 25-26 inches in circumference, weigh 12-13 ounces, and should be inflated to 9 PSI.
There is nothing in the rules stating the diameter of a ball, but the circumference must be between 68-70cm/27-28inches.