Jamieson, Fausset and Brown on Isaiah 40:12

12. Lest the Jews should suppose that He who was just before described as a "shepherd" is a mere man, He is now described as God.

Who--Who else but God could do so? Therefore, though the redemption and restoration of His people, foretold here, was a work beyond man's power, they should not doubt its fulfilment since all things are possible to Him who can accurately regulate the proportion of the waters as if He had measured them with His hand (compare Is 40:15). But Maurer translates: "Who can measure," &c., that is, How immeasurable are the works of God? The former is a better explanation (Job 28:25; Pr 30:4).

span--the space from the end of the thumb to the end of the middle finger extended; God measures the vast heavens as one would measure a small object with his span.

dust of the earth--All the earth is to Him but as a few grains of dust contained in a small measure (literally, "the third part of a larger measure").

hills in a balance--adjusted in their right proportions and places, as exactly as if He had weighed them out.