Typically, a manufacturer provides a size or measuring guide on its website. For the style that has arm and back flaps, there are three measurements you need. Measure the width of the seat, measure the width of one of the arms, and measure the back from top to bottom, where it hits the seat cushion. For example, if you have a sofa that measures up to 78 inches wide, you need an oversized cover. A sofa with a seat measuring up to 70 inches wide needs a standard size cover. A loveseat measuring up to 54 inches wide needs a loveseat or two-cushion cover. A chair with a seat that’s 48 inches wide needs a chair-and-a-half size. A standard chair that’s 23 inches wide needs a regular chair size.