Galatians 6:11

The word "large" [πηλίκοις] does not indicate size, but rather the irregularity of the letters. As if he said: And although I know how to write these things well, nevertheless I have been forced to write the letter with my own hand, because, he says, my confession is set down in my own hand, that I do not preach circumcision as those who slanderously say.

Those who wanted the Galatians to be circumcised had put it about that Paul preached one way and acted another, destroying his words by his own deeds, since he who proclaimed the abolition of the law was found to be obeying the law. Because Paul could not refute their opinions in person in the sight of all (being prevented by the chains that he bore as a testimony to Christ), he acts as his own lawyer through his letter. So that no suspicion that the letter was false might arise, he himself has written it from this point right to the end, showing that the preceding part was copied by another's hand. … It is not that the letters were larger (though indeed the word would bear this sense in Greek) but because the marks of his own handwriting were known to them. So when they recognized the angles and contours of his own letters, they would feel that they had encountered him.… Paul wrote his letter in great characters because the meaning of the characters was great and had been traced out by the Spirit of God, not merely by pen and ink.

"See with how large letters I have written unto you with mine own hand. As many as desire to make a fair show in the flesh, they compel you to be circumcised." Observe what grief posesses his blessed soul. As those who are oppressed with some sorrow, who have lost one of their own kindred, and suffered an unexpected calamity, rest neither by night nor day, because their grief besieges their soul, so the blessed Paul, after a short moral discourse, returns again to that former subject which chiefly disturbed his mind, saying as follows: "see with how large letters I have written unto you with mine own hand." By this he signifies that he had written the whole letter himself, which was a proof of great sincerity. In his other Epistles he himself only dictated, another wrote, as is plain from the Epistle to the Romans, for at its close it is said, "I Tertius, who write the Epistle, salute you;" (Rom. xvi: 22.) but in this instance he wrote the whole himself. And this he did by necessity, not from affection merely, but in order to remove an injurious suspicion. Being charged with acts wherein he had no part, and being reported to preach Circumcision yet to pretend to preach it not, he was compelled to write the Epistle with his own hand, thus laying up beforehand a written testimony. By the expression "what sized," he appears to me to signify, not the magnitude, but, the misshapen appearance of the letters, as if he had said, "Although not well skilled in writing, I have been compelled to write with my own hand to stop the mouth of these traducers."