You need to measure the brass before and after sizing so you know where you stand. After that it should give you a idea of what you need.

Is the shell holder touching the bottom of the sizing die? Is the press camming over during your sizing? I don't let the ram cam over when sizing. I make contact with the die just before cam over to get the most leverage when sizing.

Small base die for the win! Brass cycles fine now. Now back to the fun stuff.

I'm glad that's it. Sounds like you had it nailed from the start.