Bestmastertailor I think was the one who said that as a general rule DB suits should be cut slightly longer. I think it had something to do with the fact that the extra length (I think he was talking about 1/4" -- so not much) is to balance out the added width that the double breasted suit gives the illusion of.
I recall that A. Harris was also confused as to Bestmastertailor's theory and accepted it with a mere shrug of the shoulders. Personally, I don't think there should be any rules about length here, and that it will be in the eye of the beholder about what looks better (subject to some objective criteria, such as the "cover the seat" rule). Indeed, Bestmastertailor was not proposing any rule per se, but offering an observation that a longer DB jacket will LOOK better because of the need to balance out the width.
I agree with Renault78law: DB coats should, all other things being equal, be slightly shorter. Â It's basically a matter of the optical illusion presented. Â Because of its wrap and almost dead-straight bottom edge, a DB coat "envelops" the wearer and appears longer than an SB coat of the same length. Â Because there are no open quarters, or curved front bottom corners, you don't see the trousers at all until they emerge from under that straight bottom edge. Â Thus the leg line appears shorter, and the trunk longer.
Good reasoning, and I would concur with it. Again, it's all about the illusion that the db creates. So, my advice is first to listen to Manton, and second to try on a db coat and decide for yourself whether a shorter or longer coat looks better.
The key to what Bestmastertailor was saying is the button stance - it is low on his double breasted suits (which are very likely worn fastened on the bottom button) to disguise a generous midsection. That is very effective if done correctly, but necessitates a longer jacket.
If the drop is more standard, and especially if worn fastened on the middle button, a shorter jacket looks better, I stand by that.