Protists are a type of unicellular organisms that are eukaryotic which means they contain a defined nucleus but that are not plants, animals or fungus. Considering protists only contain one cell in most of the cases they are small or restricted in size this includes Dictyostelium which are microscopic organisms that consume bacteria and plasmodium. However, in the case of Caulerpa, these are not limited to size as Caulerpa is a type of alga that has macroscopic cells and indeed in some cases can be around 3 meters long. On the other hand, Ulva is also an alga but is considered in the plants' domain and therefore is not a protist. Therefore the one that contradicts contradict the statement that unicellularity restricts cell size in protists is the Caulerpa.
