On the wikipedia page of crinoids (sea lilies and feather stars), one can find the following statement in the paragraph "Morphology":
[...] and fossil species are known with 20 m (66 ft) stems, the largest recorded crinoid having a stem 40 m (130 ft) in length.
They also give some sources for this, but they also just mention the fact without specifying the fossil/species. When searching on the internet about the largest known crinoid species, one can find very often similar claims, but again without any source (probably coming from wikipedia in the end). In general, I am slightly doubting the statement about 40m of length: Statements about size of large organisms are very often exaggerated. For example, in popular science books, one can often find lengths for the giant squid (Architeuthis dux) of about 20m, while the truely largest individual every measured with scientific methods was probably "only" about 12m (see the paper here for example), which is of course still pretty impressive. In the words of S. J. Gould: "Our strong and biased predilection for focusing on extremes generates all manner of deep and stubborn errors".
Anyway, I would like to know how large/tall crinoids can get by knowing how large the largest species ever to be found was (preferably with a scientific resource supporting the claim). On my internet research, I found the species Seirocrinus subangularis (Miller, 1821) from the early Jurassic with stem lengths of about 20m, although also here the statements about size vary a lot from 15 to 25m and I am not able to find a precise, confirmed, statement.