By definition, a volcano is a conical structure that builds around a volcanic vent. A volcanic vent is an opening from which molten rock and gases erupt. According to the definition, there is no size requirement. There are volcanoes that you can span with your legs while on the other hand, there is Mt. Fiji.
Renton, John J. and Repine, Thomas, "Kinds of Volcanoes" (2016). Readings and Notes. 10.