What powers do the two witnesses have?
The two witnesses are given power to shut up heaven so it does not rain, to turn water into blood, and to strike the earth with any kind of plague as often as they wish, and fire comes out of their mouths to consume anyone who tries to harm them.
Revelation 11 tells us that God gives His two witnesses remarkable power during their 1,260 days (three and a half years) of prophesying in sackcloth. First, they have power to shut heaven so that it does not rain during the days of their prophecy. Second, they have power over waters to turn them to blood, and to smite the earth with all plagues as often as they will. Third, if anyone tries to hurt them, fire proceeds out of their mouths and devours their enemies, and anyone who attempts to harm them must be killed in that same manner.
These powers are shown to echo the ministries of two great prophets of old. The shutting of heaven so it will not rain recalls Elijah, who declared to Ahab that there would be no dew or rain except by his word. The turning of water to blood recalls Moses, who struck the river in Egypt and turned its waters to blood before Pharaoh. The two witnesses are also identified as the two olive trees and the two candlesticks standing before the God of the earth, a picture drawn from Zechariah's vision of the two anointed ones who stand by the Lord of the whole earth.
Even after finishing their testimony, when the beast from the bottomless pit overcomes and kills them, their power is not truly ended, for after three and a half days the Spirit of life from God enters them again, they stand up, and a voice from heaven calls them up to heaven in a cloud while their enemies watch. Their ministry, their protection, their plagues, and even their resurrection all point to God's sovereign power at work through them even amid great rebellion on the earth.
The scriptures
And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days, clothed in sackcloth. These are the two olive trees, and the two candlesticks standing before the God of the earth. And if any man will hurt them, fire proceedeth out of their mouth, and devoureth their enemies: and if any man will hurt them, he must in this manner be killed. These have power to shut heaven, that it rain not in the days of their prophecy: and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will. And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them, and kill them. And their dead bodies shall lie in the street of the great city, which spiritually is called Sodom and Egypt, where also our Lord was crucified. And they of the people and kindreds and tongues and nations shall see their dead bodies three days and an half, and shall not suffer their dead bodies to be put in graves. And they that dwell upon the earth shall rejoice over them, and make merry, and shall send gifts one to another; because these two prophets tormented them that dwelt on the earth. And after three days and an half the Spirit of life from God entered into them, and they stood upon their feet; and great fear fell upon them which saw them. And they heard a great voice from heaven saying unto them, Come up hither. And they ascended up to heaven in a cloud; and their enemies beheld them. And the same hour was there a great earthquake, and the tenth part of the city fell, and in the earthquake were slain of men seven thousand: and the remnant were affrighted, and gave glory to the God of heaven.
Then answered I, and said unto him, What are these two olive trees upon the right side of the candlestick and upon the left side thereof? And I answered again, and said unto him, What be these two olive branches which through the two golden pipes empty the golden oil out of themselves? And he answered me and said, Knowest thou not what these be? And I said, No, my lord. Then said he, These are the two anointed ones, that stand by the LORD of the whole earth.
And Elijah the Tishbite, who was of the inhabitants of Gilead, said unto Ahab, As the LORD God of Israel liveth, before whom I stand, there shall not be dew nor rain these years, but according to my word.
And Moses and Aaron did so, as the LORD commanded; and he lifted up the rod, and smote the waters that were in the river, in the sight of Pharaoh, and in the sight of his servants; and all the waters that were in the river were turned to blood.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.