I bought the NB stir plate and am using it for the third time. Because I’m a typical guy (my opinion) bigger is always better, and new and improved is just that because an advertiser would never lie to me. The smaller stir bar that came with the NB plate seems to find the magnets on the plate easier, makes a larger vortex, and is less noisy than the larger stir bar with the pivot ring. Just an observation.
I’ve noticed the same with my non-NB stir plate. I bought a bigger magnet thinking it would work better and found it is much easier to throw off than the smaller one. I have a 2L flask and a 1gallon jug. Both work on the stir plate (yes the jug is a pain in the a$$ to get the magnet up and working due to the indentation on the bottom, it’s like a mini plateau), but the smaller magnet works great in both. The larger one, not so much.
This is because the short stir bar sits inside the magnetic field generated by the driving magnets. The edge of the longer bar will sit on the edge of the magnetic field and can easily be knocked out. When designing your stir plate, you can account for this by making your drive magnet circle diameter bigger than your desired stir bar length. Mine works just as well with a 1.5" and 0.5" bar, for example. If your stir plate came with a bar, this is probably the length for which it was designed to work.
Longer stir bars will be able to move more liquid for a given rotational speed, but also requires more input torque. You might find you need more liquid moving for 4L+ starters, and this can be done by either spinning a smaller bar faster or spinning a larger bar. The spacing from stir bar to magnet is important: too far and the bar won’t get enough magnetic force to spin, too close and the magnet will pull the bar down into the vessel and create excess friction.
Anyway, it’s not really important how you get there, as long as you get a slight depression on the wort surface.
I bought the 2 inch stir bar and I think I actually invented new swear words while trying to get it to work. The 1 inch works great.
See boys? Size isn’t everything. 
