That's a great question, and one that I've figured out through a bit of trial and error in my own apartment! For a standard-sized urban trash can—the kind you'd find in most kitchens—the perfect liner is almost always a 13-gallon trash bag.
These "Tall Kitchen" bags are specifically designed to fit the classic, roughly 10-inch by 20-inch can. The key is the fit. A bag that's too small will slip down inside, and one that's too large is wasteful and looks messy. A 13-gallon bag typically has a slight slack when first placed in the empty can, but its drawstring or flap-tie handles make it easy to secure over the rim. This prevents it from falling in and makes taking the full bag out a clean, simple process.
When shopping, look for bags labeled for "standard" or "kitchen" use. I personally prefer bags with drawstrings for a tight seal, especially for food waste, as it helps contain odors. Also, consider the thickness or "gauge." A mid-range thickness is ideal; it's strong enough to handle wet coffee grounds or a broken glass jar without tearing, but not so thick that it's unnecessarily expensive or stiff.
So, to sum it up, grab a box of 13-gallon bags. They are the gold-standard liner for that common urban kitchen can, providing a reliable, leak-resistant, and convenient solution for daily waste.