1 Tefach is just under 4 inches. 18 Tefachim = 66 inches.
Please cite any source to the allegation that either Rav Soloveitchik held 3 feet or 33 inches (as two conflicting claims above make) and which Rav Soloveitchik (another conflicting claim above between Feif and tsharer), as I believe no source for any such claim will be found.