There are several definitions in use, but a common one is that any particle with at least one dimension less than a micron is a nanoparticle. An alternative definition is ‘particles with all three dimensions less than 100 nm'. Nanoparticles may be of any shape (flat, rod’shaped or spherical).
Note that a micron is one millionth of a metre, and a nanometre is one thousandth of a micro (i.e. there are one thousand million nanometers in a meter). For comparison, a human hair is about sixty microns thick.
For comparison, viruses vary in size from a few tens of nanometers to hundreds of nanometres. Bacteria also vary in size, but are much larger than viruses with typical sizes greater than one micron (or one thousand nanometres).