If you are unable to get the model number you can measure from the center of the right arm to the center of the left arm, DO NOT take the measurements of the old fabric or roller tube.
Per the manufacturer these awning replacement fabrics are hand rolled and put in tubes rather than boxed, avoiding wrinkling and creasing to the fabric. These Dometic replacement fabrics will ship via freight truck, so please plan accordingly.
"Fabric length" is based off of the length of the whole awning (i.e. 834XX15.XXX) which will determine the "size" of the fabric needed. The actual fabric will almost always be a smaller measurement (by at least 6 inches) than the awning length.