Final answer:
Viral dimensions are typically measured in nanometers, requiring the use of an electron microscope due to their ultramicroscopic size.
Step-by-step explanation:
The dimensions of a virus are usually described in nanometers. Viruses are ultramicroscopic entities, with the majority of virus particles, known as virions, falling within the range of about 20 to 250 nanometers in diameter.
However, some larger viruses can measure up to around 900 nanometers or even 1000 nanometers for certain recently discovered giant viruses. To observe these small sizes and the detailed structure of virus particles, researchers use the electron microscope.