So if you're reactor is on the scale of meters the neutron length scales, it looks like a thermal neutron, is in the order of a tenth of a meter.

The standard meter fulfilled its duty perfectly, although its length has been found much later too short by 0.23mm with respect with the meridian.

I make it very simple — I make the length one meter, and I hang on the rod, at the bottom, a mass m, which, let's say, is 500 kilograms.