Thanks for the reference, pster.
I think the way LSJ lists the entries is first giving the common form, which in most cases is Attic,
then the Ionic and other dialects if there is any difference. Here we have the Attic form,
μέγεθος, and then the Ionic one [Herodotus, though not Hp.(? can't find this abbreviation
I find only μεγέθεος as the Ionic gen. sg., not μεγάθεος,
whereas Plato had used the contracted Attic μεγέθους.