An empty lunch bag can look roomy until the food container goes in. Begin with the container you already use, then add the cold sources required for perishable food. Measure that complete arrangement before choosing a bag by its advertised capacity.
Think about how you travel. A bag carried separately may benefit from a comfortable handle and a stable base. One going inside a backpack needs to fit without forcing books or a laptop against the food. A tall narrow design is not helpful if your usual lunch has to remain flat.
Insulation is only part of cold storage
An insulated bag slows temperature change; it does not make warm food cold or keep perishables safe indefinitely. Start with chilled food and use adequate cold sources. USDA guidance recommends at least two cold sources for a packed perishable lunch, placed to help keep the food cold.
If a refrigerator is available when you arrive, use it. Consider the whole gap between leaving home and eating, including an unexpected late class or a warm bus ride. A small refrigerator thermometer or other suitable temperature check can be more informative than judging safety by the bag's outer surface.
Do not crowd the bag so tightly that the closure cannot shut. Check that frozen packs fit without crushing a sandwich or pressing against a fragile lid. Separate a drink if its shape makes the food arrangement unstable.
The inside should be easy to clean
Inspect corners, seams and the lining. Food spills need to be removable using the manufacturer's cleaning method. A bag that cannot dry fully between uses may become unpleasant even if it initially looks tidy.
Look for a practical place for cutlery and a napkin, but avoid paying for pockets you cannot use. External pockets do not necessarily provide the same insulation as the main compartment. Keep perishable items in the part designed for temperature control.
After lunch, empty the bag instead of leaving wrappers and containers inside until the next morning. Clean spills promptly and let the interior dry open. The best size is often just large enough for your normal lunch and cooling arrangement, with a little flexibility for the days that run longer.