Well, how do we know that the proton has a fm? When hitting it with a charged particle with a Compton wavelength in the order of 1 fm in a scatter experiment, we start to see a formfactor, e.g. the particle doesn't see the full charge any more.
LEP II had about 100 GeV/c^2 per electron, therefore a Compton wavelength of about 10^(-17)m, and there was not the slightest sign of substructure, we still see absolutely the full structure and one would likely expect already small signs of a formfactor with a somewhat bigger wavelength than the electron size.

I don't know what measurement exactly provides the best measurement, could be a precision measurement of the B sector or whatever, but that's it pretty much.

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Volker Pispers