One should be able to measure it from a model editor as I think WC3 models are scaled 1:1 with WC3 in game units.
A rather clumsy way, but more scientific than guessing, is to place the unit at scale 1 with fly height 0 in the editor and measure where the visual weapon is located using the in game grid. This approach should allow one to get good values on the first try, with maybe only a few tests needed to align them perfectly.
In WorldEdit, each big grid is 512*512, the standard grids are 128*128 (1 tile squared) and the small grids are 32*32.