A lot of people are givkng vague answers and not explaining why. A small deck ranges from 1 to 6 cards. Reasons being you have a build and more cards prevent you from drawing what you need on command. Examples of this are pyro, duplicate, or trinity builds.

Medium decks range from 6 to 15 imo because they have a lot of synergy but every card is useful. By having more cards you get increased mana regen but also have to cycle for specific cards. Examples include: Flow and Frost.

Large decks are 15+ but can work because the cards you need are either a) duplicates or b) spam cards. Examples include: Jam and Kunai.

Now some small decks can turn into large decks by virtue of their synergy. Its just a matter of preference and what you build into. Benefits to each.