The Everyday Life of Abraham Lincoln Frances Fisher Brown repeats a story told her by a Senator McDonald, who witnessed the trial:
“No blood had been spilled, but there was malice in the prosecution, and the chief witness was eager to make the most of it. On cross-examination, Lincoln “gave him rope” and drew him out; asked him how long the fight lasted and how much ground it covered. The witness thought the fight must have lasted half an hour and covered an acre of ground. Lincoln called his attention to the fact that nobody was hurt, and then with an inimitable air asked him if he didn't think it was ‘a mighty small crop for an acre of ground.’ The jury rejected the prosecution's claim.”
Interestingly enough, we have a description of the trial given by Lincoln himself. He was visiting a cousin in Cincinnati at the time. His cousin was married to a judge, and apparently Lincoln couldn’t resist the temptation to relate a “war story” to a fellow member of the bar. The judge then told the story to Joseph H. Barrett, and Barrett included it in his work, Abraham Lincoln and his Presidency. There are some discrepancies between the two stories, the main one being the size of the field:
“I was retained in the defense of a man charged before a justice of the peace with assault and battery. It was in the country, and when I got to the place of trial I found the whole neighborhood excited, and the feeling was strong against my client. I saw the only way was to get up a laugh and get the people in good humor. It turned out that the prosecuting witness was talkative; he described the fight at great length; how they had fought over a field, now by the barn, again down to the creek, and over it, and so on. I asked him on cross-examination how large that field was; he said it was ten acres; he knew it was, for he and someone else had stepped it off with a pole. ‘Well, then,’ I inquired, ‘was not that the smallest crop of a fight you have ever seen raised off of ten acres?’ The hit took. The laughter was uproarious, and in half an hour the prosecuting witness was retreating amid the jeers of the crowd.”
Other biographies of Lincoln put the size of the field at six acres. A single acre is the probable size, with those repeating the story making it larger and larger. But no matter what the size of the field, Lincoln simply let the witness run with his embellishments until the witness was claiming to have measured the field. He had gently assisted the suicide of the witness, allowing the witness to become so emboldened by the soft cross-examination that he began making implausible assertions. Lincoln then, instead of savaging the witness with an aggressive challenge, pointed out the implausibility with a single question.