There is no set definition for a small apartment. But if you consider an apartment with less space than average to be small, the answer is any unit below 550 square feet.
Whether or not you consider your apartment small is subjective. Many factors can contribute to the amount of space you need and have, such as living with roommates or family, or working from home.
If your living room feels small, regardless of its actual square footage, referencing small apartment decorating ideas is sure to help at least a little.