You can measure small lengths in millimeters. Length of a pencil, pen, eraser, length and width of a paper, sheet etc. can be measured in millimeters.
It would be millimetres.
I would go with millimetres
If I was you i would use millimetres or centimetres.
I would use centimetres and millimetres
A calculator would be between 100 and 150 millimetres.
Either inches or millimetres Or centimetres.
millimetres or even micrometres.
A large beetle maybe, but generally you would measure a beetle in millimetres.