"An amoeba is not visible to the naked eye. So ''several milimeters'' is out of the question."

I should think so, too, but here's the reference, and it looks like a reliable source:

William R. Pendergras. Carolina protozoa and invertebrates manual: Carolina biological supply company, 1980.

"Most protozoans are microscopic, but certain amoebae reach up to 4 to 5 mm in diameter, and the shells of their cousins the foraminifera may be 10 cm across."

Meh. Who can be sure of anything anymore...?

Have fun!