What does Jesus mean in Matthew 26:64 with “You have said so”?

Jesus answers the question for us. When Peter proclaimed that Jesus was the Messiah, the Son of God, Jesus said only the Father revealed it to him.

   Mt 16.16 And Simon Peter answered and said, Thou art the Christ, the Son of the living God. 17 And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.

Now the high priest had not been around Jesus as much as Peter, so his direct instruction was even less than that of Peter. How did he know how to ask such a direct question? The Jews did not expect the messiah to be the Son of God [1], but a man who would free them from Rome.

   Mt 26.63 But Jesus held his peace. And the high priest answered and said unto him, I adjure thee by the living God, that thou tell us whether thou be the Christ, the Son of God.

He knew the same way that Peter did. The Father had revealed it to the High Priest, but rather than acknowledge it, he would rather kill him. [2]

   64 Jesus saith unto him, Thou hast said: ...

Jesus is saying that the High Priest has testified himself that Jesus is the Messiah, the Son of God, by merely asking him the question, that would have been out of character for a mere human understanding of what Jesus was doing.

Jesus also confimed that the priests knew the truth when he called them a brood of vipers:

   Mt 23.33 Ye serpents, ye generation of vipers, how can ye escape the damnation of hell?

The serpent is a reference to the serpent of the garden which was the father of lies.

[1] Mt 16.14 And they said, Some say that thou art John the Baptist: some, Elias; and others, Jeremias, or one of the prophets.

[2] Joh 18:14 Now Caiaphas was he, which gave counsel to the Jews, that it was expedient that one man should die for the people.