[alphydan]

Start by measuring it over a few cm. Their setup is "the same" setup Cavendish used in the 18th century (a torsion balance). Read about it here: http://www.michaelbeeson.com/interests/GreatMoments/Cavendis... or watch this beautiful video explaining the idea: https://drive.google.com/file/d/0B1NTp_zR8Gl7VTJ2aU55cWQzd2s...

That will give you a flavour of the difficulty involved.

Below those sizes, electrical forces start making your experiment very difficult (electrostatic, van de Waals), and any vibration will pollute your data. This is where the "can't do it at home" starts to read its head (something to compensate the vibration of buses driving nearby, a lab with no static electricity or magnetic fields anywhere, etc, etc)

Beyond ... the Casimir effect places a limit to the lengths you can reach.