Am I missing something?
Use a metric tape measure and measure in centimetres. The length and width are the usual measures but you might as well measure the height too.
As for decimals 100cm = 1 metre.
I don't think you would get total accuracy because the milipede has a greater inclination than the centipede to have a slight oscillation in their stance when in a queue - this I feel would slightly shorten their overall length in a measuring situation...it's a conundrum no doubt about that!
don't forget to brush off the segmented bugs first though bibblebub or your answer could be out by as much as 100gms! (4 oz to those reading in imperial weights)