The size of the gas strut is measured when the strut is fully extended. The strength (measured in lbs) is usually listed on the strut itself. If it is not or got rubbed off a rough guide is that most framed topper doors (has metal frame around glass) use a 24 to 28 lbs strut. If you have a frameless/bonded glass door (when door is shut no frame is seen around the glass) they usually use around a 35 to 40 lbs strut. If you have any questions on which strut will work for you feel free to call or email.