When brewing espresso, measuring quality by the “length of the shot” is failing to understand the purpose of controlling variables.
“Length of shot” equals “strength”. A short shot will be very strong, and a long shot will be weaker. But neither is a measure of quality. It is, theoretically, possible to pull a quality 100g shot (although current technology makes this hard eg fines).
Quality comes from “extraction percentage” or “yield”. This refers to the amount of coffee extracted from the ground coffee during brewing. The arguably agreed upon range is 18-22%. At this point there is good, complex flavour and balance.
Looking back then, a short shot and a long shot can both be deemed to be good extraction if the right yield is achieved. This is why I say it’s theoretically possible to pull a shot the size of a cup and it still be delicious. Strength is simply the concentration in which the 18-22% yield is held.
Having said that, it is true that there are also general agreed upon strengths/concentrations, but they’ll all be sub-par if we fail to achieve a quality extraction.