That's a great question! For a small apartment patio, you don't want a bin that's too bulky, but you also don't want to empty it every other day. Based on common experience and practicality, a good average capacity is between 20 to 30 liters (roughly 5 to 8 gallons).
Here’s the reasoning: A bin in this range is compact enough to fit comfortably in a limited space without dominating your patio. It's perfect for handling a week's worth of typical patio waste—like snack wrappers, empty drink bottles, or small cleaning debris—if you're out there regularly. If you entertain occasionally, a 30L bin can manage the extra party waste without overflowing immediately.
The key is to consider your habits. If you're a solo dweller who rarely uses the patio, a 20L might be plenty. For a couple or someone who enjoys frequent small gatherings, leaning toward 30L is smarter. Also, think about the type of lid (sealed to deter pests) and durable, weather-resistant materials like resin or galvanized steel. Ultimately, the "perfect" capacity balances your space constraints with how often you're willing to take the bag out.