The relative transmissions of helium dimer and helium atom beams through a set of nanoscale sieves were measured as a function of hole size in the range from 98-410 nm. From the relative transmission coefficients, the mean internuclear distance of helium dimer was determined to be (r) = 62 +/- 10 Angstrom. This enormous bond length-by far the largest known-confirms recent theoretical estimates of the potential energy well depth epsilon and the extremely small binding energy E(b) of helium dimer. The ranges of these parameters corresponding to the experimental uncertainty in (r) are epsilon/k = 10.88-10.98 K and E(b)/k = 0.65-1.30 mK, where k is the Boltzmann constant.