I'm always interested in units for measurement and was impressed to see a figure given in square millimetres. 1cm is 10mm so 1 square cm is 100 square mm. This means the area is 40 square cm. There are 1 million square mm in 1 square metre because it is 1000mm x 1000mm so the area is 0.004 square metres. It is circular so the diameter must be 40 = pi x diameter squared /4. Diameter = 7.1cm which is about right.
