The sizes of Cubesats are measured in "U" units of volume. Each U is nominally a cube of something like 10 to 12 cm on a side, depending on the specific deploying mechanism it's approved for.
1U to 3U are popular sizes, the MarCO deep space cubesats were 6U.
Question: What is the largest proper cubesat? It should be either attempted to have been deployed, or at least on track; serious, funded effort with construction or at least final design underway. Answers citing only crowdsourced project descriptions and/or theoretical papers may receive "honorable mentions" but won't be accepted.