It comes down to personal preference and what you cut. As a weekend firewooder, I regularly buck greater than 42” hardwood and frequently fall 30”+ oak with 60 to 123cc saws. I have bars from 18” to 52”. I actually use 20”, 25” and 33”. Probably used 75/20/5. Never use the 18” or longer than 36”. Hardly ever use the 36”. Long bars are nice for falling. Also a 25” or longer bar is nice with a light saw if you cut small stuff at ground level. For most firewood bucking a 20” will do it quite well.
Ron
Ron