That's a great question! For a family of four, the most common and practical recommendation is a 32 to 35-gallon outdoor trash can. This size typically hits the sweet spot for weekly municipal trash pickup schedules. It's large enough to hold a week's worth of general household waste from a family of your size—think kitchen bags, packaging, and everyday non-recyclables—without becoming overly heavy or cumbersome to move to the curb.
A 32-35 gallon can is also a standard size that fits well in most storage areas and isn't too bulky on the curb. It helps manage odors since you're not letting waste sit for too long (ideally just the week between pickups). If your family generates more waste, perhaps due to frequent cooking or hosting, you might consider stepping up to a 45 or 64-gallon can. However, these can get very heavy when full and might be excessive for a standard four-person household. The key is to match the can to your local service's capacity rules and your actual weekly output. Start with a 32-35 gallon model; you can always size up if you find yourself consistently running out of space before collection day.