Viruses are small, from about 20 nanometers to about 400 nanometers in size. (A bacterial cell is generally in the range of 0.5 to 5.0 micrometers in size. A micrometer is one thousand times bigger than a nanometer, so bacteria are hundreds of times larger than viruses.)
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Viruses cannot be killed by antibiotics. Antibiotics kill or stop the growth of bacteria, not viruses. Using antibiotics to try to control viral diseases like colds and flu just hastens the day those antibiotics will be useless against dangerous bacteria, because exposing populations of bacteria to antibiotics gives them a chance to evolve defenses against the drugs.