My 2 cents.
1" stir bar works best in 2000ml or smaller flasks. It works fine most of the time in my 5000ml flask except with highly flocculent yeasts like wheat because the yeast will form large clumps and settle towards the outside of the flask where the liquid is not rotating much. When I turn up the speed all the way the wort is rotating very fast towards the center but is barely rotating towards the outside.
The 2" stir bar creates a wider column of rotating wort to help keep yeast clumps from settling towards the outsides of the flask, which also prevents larger clumps from forming I'm the first place. However, I do have a harder time keeping the 2" bar from being thrown but I can get it going fast enough to create a small vortex which is all you need. My 1" stir bar can hold on no matter how high I turn up the speed.
My stir plate uses cylindrical magnets. I recently replaced 1 of the failing drives in my NAS so I now have a couple of hard drive magnets waiting for me to try out to see if they work better for the 2" stir bar.