The smallest cell ever discovered is called Mycoplasma gallicepticum, (bacteria belonging to the Mollicute class), a parasitic organism that can cause a diverse amount of respiratory chronic deseases in all sorts of birds. These species have a minimum size of 0.1μ.
Remember, this is the smallest cell ever found to this day, but there might be smaller ones yet to be discovered. And you shouldn't doubt on this, since any cell smaller then the Mycoplasma gallicepticum would be very small, and hence the difficulty to be discovered.
Important note:
In a test, if they ask for this bacteria, you should specify the species (gallicepticum), since there are dozens of Mycoplasma species.
Hope it helped,
BioTeacher101