There are 25.4 mm in one inch, so 6 mm is about 1/4 inch.

9x7 is a propellor size. It means it's 9 inches from tip to tip, and it will theoretically move forwards 7 inches per revolution. (Imagine a bolt turning one revolution in a nut, and the amount it moves axially is the pitch)

'9' is called the diameter, '7' is called the pitch.

So, a 16x4 is 16 inches from tip to tip, and it will move forwards (theoretically) 4 inches per revolution.

-David C.