I've heard that extra dimensions can be (due to gravitational experiments) of dimensions ~some millimeters... I'm quiet skeptical about this issue... Why would someone choose gravity (such a weak force for those experiments) to determine the scale? why not use Electromagnetism , that has allowed us to see up to ~some Armstrong scale (seen the atoms)? with the last, we know that we haven't seen any extra dimensions up to that scale... Moreover on that, if the extra dimensions were really of order of some millimeters, then a dust of dirt could be completely erased from our world - being able to travel within those dimensions. It doesn't sound nice.... I am sorry if this belongs to Beyond SM, but it's not about "research".