This is a big question because of the wide variance of individual strength levels. Fortunately a 41 Inch standard size. Our Band provides a resistance variance of at least 30 regardless which band you choose.
For example, a black medium band will create a resistance between 30 to 60 lbs depending on the starting tension and end of movement tension. Comparing this to constant free weight resistance that has only 1 resistance level it provides the user with multiple levels of resistance in a single band. That said, we still recommend purchasing at least 2 or 3 levels of resistance bands but that is a whole lot less expensive and more space efficient than having to purchase a dozen pairs of dumbbells.