When I looked for the largest protozoan I've found the name Syringammina fragilissima. According to wikipedia " .. Syringammina fragilissima, is among the largest known coenocytes, reaching up to 20 centimetres (7.9 in) in diameter." The catch is it is multi-nucleated organism, not exactly what I had in mind. The reason I am interested in the upper limit of the size of the free living unicellular organism is because I want to have a sense of the limitation of single center has in emitting and receiving information across a large span of space. My interest is not about non-compartmentalization (as in the case of multi-nucleated unicellularity) rather about single center.