Heating oil tanks come in different sizes, but a common size for residential tanks is 275 gallons. However, tanks can range in capacity from as small as 100 gallons to larger sizes like 500 gallons or more. The specific size of the tank depends on factors such as the heating needs of the property and available space for installation. It's important to note that the capacity of the tank should comply with local regulations and meet the heating requirements of the property. Consulting with a heating professional can help determine the appropriate tank size for your specific needs.