The field of view at 40 power is about 4 mm wide.

The grey object is less than 1 mm in length.

(It would take more than four of the grey objects to fit across the field of view.)

The actual size of the grey object is still the same--  less than 1 mm in length, but now we are able to estimate its size better - about 0.8 mm. (because it reaches about halfway across the 1.6mm field of view)

The actual size of the grey object is still the same--  less than 1 mm in length, but now we would not be able to estimate its size better, because the entire object would be twice as wide as the field of view, and we wouldn't be able to see the whole object at one time.