I'm no pro, but I like bars a little on the long side and light. Of course skip chain helps.
Mainly for reach and not having to walk around to the other side and cut again. But I won't bury a long bar in more wood than the engine can hang with. Plus if I cut half from each side it would look like a beaver chewed it off, ha!
I hate the heavy 30" Cannon Super Bar on my 395. 24" bar bucking with 395 is fun.
A lot of it depends on the wood. I have big soft pine, big hard sticky pine, big rotten whatever, or big hard euc...
playing with the 42" (got it mostly to try milling) the other day I discovered my saw didn't get all dusty cuz it was 3' from the log! Dusty rotten desert wood, that's actually kinda nice...
42" - when you're over here and the log you want to cut is on the other side of the pile...