If you use the FineCut plug in for Illustrator or Corel, it will recommend a size for you depending on how big the job is. Otherwise most people use the default. If you're getting mark detect errors, try upping the sensor sensitivity. It will be in the regular cutting menu on the printer. Also, when setting the red dot to detect the marks, make sure you aren't touching any of the mark. If the dot it touching any of the black part of the mark, it can miss the detection. The best way to do it is to imagine the mark is a complete box and put the dot in the middle of that box. A lot of people try to get it right in the corner for some reason and that causes issues.