What limits does the right to a trial by jury place on the monarch?

The right to a trial by jury basically prevents the Monarch from choosing the verdict. That is the limitation that the Monarch will experience in that type of trial.

In Modern juries, the Monarch was the one making the final decision, deciding whether or not the accused person was guilty. Often times the Monarch was highly impartial and would dismiss the evidence provided in the case. So the verdict could be considered faulty or unfair.

In 1215 trial by jury became right in order to prevents those things from happening. Article 39 of Magna Carta says that every free man has the right to a lawful judgment of his peers.