Recent work considered the ultimate (quantum) limit of the precision of estimating the distance between two point objects. It was shown that the performance gap between the quantum limit and that of ideal continuum image-plane direct detection is the largest for highly sub-Rayleigh separation of the objects, and that a pre-detection mode sorting could attain the quantum limit. Here we extend this to a more general problem of estimating the length of an incoherently radiating extended (line)

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... ct. We find, as expected by the Rayleigh criterion, the Fisher information (FI) per integrated photon vanishes in the limit of small length for ideal image plane direct detection. Conversely, for a Hermite-Gaussian (HG) pre-detection mode sorter, this normalized FI does not decrease with decreasing object length, similar to the two point object case. However, unlike in the two-object problem, the FI per photon of both detection strategies gradually decreases as the object length greatly exceeds the Rayleigh limit, due to the relative inefficiency of information provided by photons emanating from near the center of the object about its length. We evaluate the quantum Fisher information per unit integrated photons and find that the HG mode sorter exactly achieves this limit at all values of the object length. Further, a simple binary mode sorter maintains the advantage of the full mode sorter at highly sub-Rayleigh length. In addition to this FI analysis, we quantify improvement in terms of the actual mean squared error of the length estimate. Finally, we consider the effect of imperfect mode sorting, and show that the performance improvement over direct detection is robust over a range of sub-Rayleigh lengths.