Do shell length and width scale proportionally (isometrically)? Length and width are measured in the same units (cm) or you can think of it as cmiand cm:. That means that if the length and width increase at the same rate....there is a 1 to 1 relationship....the slope of a graph of length versus width will equal: 1 if the relationship is isometric. If the slope is not equal to 1, then the clams exhibit allometric growth with respect to length and width. Either length increases faster than width or width increases faster than length meaning that the shape changes as the clam grows