Any cell smaller than a large bacteria will be too small to resolve with a standard optical microscope. This includes viruses (all sizes), small bacteria (e.g., E. coli) and prokaryotes.
A typical animal cell is 10-40 `\mu`m in diameter, about five times smaller than what the human eye can see. A standard visible light (optical) microscope can resolve objects to about 0.5 `\mu`m in size. This is about the size of large bacteria and mitochondria. Scanning electron microscopes (SEM) can resolve details down to ~0.01 `\mu `m. In order to distinguish parts of cells SEM is nearly always implemented.
The image (under), or this interactive cell scale slide gives you an idea of the types of cells that would be too small for a standard microscope (not SEM). This website features an interactive animation that compares cells to the size of a pin head.
Image credit: Candela Open Courses