Does a virus have a well defined shape?
Some parts of viruses are consistently shaped, in particular anything that is made of multiple copies of a protein sticking together. There are methods to determine the structures of these regular components to high precision, like single particle analysis (eg. zika) and X-ray crystallography (bacteriophage HK97). Other parts of the virus are less consistent, either because they are flexible or because they are produced slightly differently each time. The nucleic acid core is often arranged semi-randomly, and the lipid envelope of many viruses can vary greatly, dependent on the process by which parts of the host cell's membrane is pinched off to create new viral particles. Here's an example of pretty extreme variability of influenza particles, based on what cell type they have infected. Virus particles with different shapes can be just as infectious, but if they are too large the number of new viruses produced by an infected cell may be too low.