bushing in the "comp seater" die? Are you wanting to know what neck size bushing? If so it will be dependent on what brass you have and how much tension you want.
There are no bushings used in the seating die that I'm aware of? Their Type-S and Comp Neck Sizing dies use the bushings and would be determined by the loaded round neck diameter and subtracting .001-.002 (or however much tension you want).
Measure the OD of a loaded round with the brass you plan to use, should be in the neighborhood of .365-.370ish. Subtract a couple thou from that measurement and that's the bushing Id start with. You may want a couple of them to experiment with.