The length of the port as a percentage of the wavelength corresponding to the tuning frequency of that port.
A longer port is larger and has a better efficiency. The larger the port, the smaller the portion of the chamber's air mass that is exchanged, and the shorter the air movement within the port, resulting in less energy loss. Of course, at some point, a limit is reached, and the phase shift of the port starts to have a more negative impact.
At lower frequencies, the wavelength is longer, so the port can also be longer. This percentage-of-wavelength approach conveniently relates the port to the desired tuning frequency.
As a general rule, a longer port provides better peak sensitivity, while a shorter port results in a broader bandwidth.

Good boxes have often been seen with ports in the 3%-6% range.
On the other hand, in purely peak-focused enclosures, ports as large as 10%-15% are very common.