Length is a weird thing. Commenters have their own ways of assessing it based on things like where the hem is in relation to the wearer's hands and such, but that is inaccurate due to some people have shorter or longer arms and hands. Also people forget that some people have longer or shorter torsos, the same for people's legs. To really assess length you have to look at the whole picture, the entire height of the person and even how fitted the garment is to assess it.
Flusser’s worth quoting on the length issue, because what he has to say is pretty much what a good tailor would say:
“The principal criterion governing a jacket’s length is that it be long enough to cover the curvature of the buttocks while giving the leg as long a line as possible […] Two methods for determining the correct method originated with America’s development of ready-made men’s clothing, which needed general guidelines upon which to establish its standards of fit. The first employs the arm as a guide; when your hand is dropped at you side, the bottom of the jacket is supposed to line up with the [knuckle of the] outstretched thumb. Though generally reliable, this formula has one drawback: arm length varies from person to person. The second approach measures the distance from the jacket’s back collar (the point at which it joins the coat’s body) to the floor, which is then divided in half. This is the procedure taught in most tailoring schools. Either of these two approaches can be influenced by dimensions unique to the wearer; a top tailor will use neither, trusting his practiced eye to take in the whole picture before deciding on the jacket’s ideal length.” — Dressing the Man, pp. 40-41.
Notice, please: (1) the first criterion is that the buttocks be covered; (2) the most frequently cited criterion, the thumb knuckle, is actually the least reliable and has its origins not in tailoring but in marketing; (3) the half-way-to-the-floor criterion is one taught to tailors (but only as a first approximation subject to adjustments to accommodate the individual).
Since most photos on SF are from the front, it’s not always possible to know with certainty if a jacket is properly covering the buttocks. A reasonable substitute is to see if it covers the crotch: does it extend to the point in the vertical dimension where the two trouser inseams meet.
The most important point, however, is the one I put in bold type (the same can be said for buttoning points, but that’s a different discussion). One dresses an individual, not an ideal.