I always figured that Paul's "large letters" language referred to the fact that he was illiterate. The fact that what he was saying was being scripted by a scribe and then he'd, so the churches he was writing to would know it was him, would script a couple of lines in his own hand.
Just what I was taught in my Christianity classes...
There is no possibility that Paul was illiterate.
He was a student of the famous rabbi Gamaliel, and one of the kings before whom he testifed (Festus?) said to him, "Paul, your GREAT LEARNING has made you mad."
Paul's family specifically sent him from Turkey to Jerusalem to study.