Find the specific density per unit volume of the particular potato cultivar you are planning to use. Then dig out your calculus book and find the formula and technique for determining the volume of a potato. Once the volume is found, then simply divide the volume by the density to derive the weight, as in W=mass/volume. Then select sufficient potatoes to fill the correct voume, before cubing of course.
Or, as was stated above, weigh a couple of spuds, peel them, and dice them up.
Seeeeeeya; Goodweed of the North
(Oh, and that smilee that's being conked on the head, that's not you, but me for being so silly.)