The might of Ozymandias in Shelley's poem is conveyed through the sheer size of the statue. It is described as "vast" and "colossal," which represents how vast and colossal Ozymandias' power was when he reigned.
The words inscribed on the statue's pedestal also allude to Ozymandias' power or might. He describes himself as "King of Kings," meaning that he is the most powerful of, or more powerful than, other kings. He is above them all, and thus the most powerful or the mightiest. The second part of the inscription on the pedestal directs all of those other, less mighty kings to "Look on (his) Works . . . and despair!" The implication here is that his "Works" are greater than all of theirs, and that they have no hope of ever being as mighty as him. His might, then, is the might of a God. He is omnipotent and, given the massive size of the statue, omnipresent.
However, although Ozymandias was once powerful and mighty, the most accurate description of his might at the time of the poem is that it is forgotten, that it has come to nothing, and that it was perhaps all in vain. Ozymandias's might, represented by the statue, is now "trunkless," and the head lies "sunk" and "shattered" in the sand of a "boundless and bare" desert. Nobody except for a traveler in a distant land even happens upon it, and because it now lies broken and surrounded by "Nothing" but "lone and level sands (which) stretch far away," the expressions of might inscribed on the pedestal seems, in hindsight, a little ridiculous and more than a little pompous.