What items can you measure in millimeter?

Answer 1

A millimetre is a small unit of measurement - about the size of a pin head so things of that sort of size are best measured using mm. Insects are a good example.

However, precision is also important factor. You will find that the sizes of electrical appliances, built in furniture etc is often given in mm.

Answer 2

You can basically measure anything in millimeters, though finding something reasonable to measure is a little hard. Measuring a wire in millimeters is reasonable, for example.