Optimumly the bigger the gap the bigger the spark.
However eventually you reach a point where the ignition system can't maintain the spark over a wide gap because there is just too much resistance in the form of empty space. The stronger the ignition in regards to both voltage and amperage the more gap it can tolerate. Volts is the push to jump the gap and amps is the volume that decides just how strong that spark is.
.045" should be right or you could go with the MSD spec of .050". Wider gap will usually give a little better idle quality.
