I have abandoned magnetic stirrers entirely. I hate it when the stir bar starts running wildly all over the container while I am in the middle of a precipitation ! I have two overhead mixers. one is for viscous mixes. High torque,low speed. The other will go to 1500rpm, but will not handle viscous mixes. So I usually switch over, just before precipitation. My mixing blades are Jiffy Stirrers. These have 2 horizontal blades connected by 2 vertical blades. These come in various radii. Mine are 2 and 3 inches.
My containers are 1-2 inches wider than the mixing blade.
At the risk of being ridiculed, I will confess a practice that I have adopted to minimize air while mixing. Early in the process, just after I have added the halide salts, I add a drop of Triton X-200. This releases much of the air introduced by mixing, reducing volume and turbulence.
Bill