I started with a 16" blade, then progressed to a 24", several 30", several 42", a 44" and final 60's.
I find myself using mainly the 24, 42 or 60 bars. Even if I'm milling a 20" max diameter log I don't bother to change to the 30 or 24" bar. I also think that having a ft or so out of the cut helps keep the bar and chain a bit cooler than burying the bar in wood all the time.
What would be nicer than swapping bars and rails is another medium size mill that I can use with the 660 or 441.