Long enough that they don't drag on the grass and short enough that they don't drag on the grass. At some ranges that means fairly short tails. I carry some spare flags on short poles in case I have to shoot on the left end at Fairchance. These have tails about 18 inches long. My normal sail tails are about 30 inches long.
Remember IR50 rules mandate flags be below a line between the bottom of the target and top of the bench. Some places you need to make adjustments to stay within the rules. I've been known to borrow scissors and amputate some from the tails to stay legal. bob
I understand about the rules for flag placement and height, plus you don't want them in the grass.
I'm wondering if a given length works better that others due to the length (weight), is more responsive, quicker vs. slower,
more stable or provide a better indication of velocity. Am I looking at it all wrong?
Yup, you are. The advice you've received is from some very accomplished veteran shooters. There is no magic length. The thing you need to do is learn what the flags are telling you, regardless of their length. Keep a pair of scissors in your shooting bag/box, trim as necessary.