See what large letters I use as I write to you with my own hand!This seems like a strange insertion into the letter, since Paul made it clear earlier that he was the writer of Galatians in chapter 1. Why bother to note the large letters, if Paul has been writing the whole time?
Galatians 6:11
The answer is that while Paul is the author of Galatians, that does not mean he actually put the pen to paper. It was common in ancient times for a letter to be dictated by an author to a scribe. The scribe was often a person who took down letters professionally, since in those days there were no printing presses or printers to produce legible results. Scribes had excellent handwriting to ensure the eventual reader would have no problems. The scribe might also be an expert in the Greek language that the Bible was written in, who could correct certain grammatical errors that might pop up. It is believed Titus served as the scribe for many of Paul's letters.
It is believed that in the case of Galatians, a scribe took down Paul's dictation except for this little bit where Paul wrote out the letter himself. And why would the letters be so big? There are two explanations commonly given. First, big letters are easier to read, even for one with messy handwriting. Second, tradition states that Paul has poor eyesight, so he would have had to write lage letters to be able to see his own work.