So, the ffd box can fit a 3d object within a 3d space by fist, creating a primitive with the specified measurements and then using the align sub-object selection to position the control points of FFD box within that space of the primitive you specifically scaled. Image Source : https://www.youtube.com/watch?v=j4fhjMJUyS0 use x, y, and z position in addition to target object options to fit your 3d object in specified dimensions Select reference primitive. > Select position > Select target box > Select "OK" Repeat these steps to adjust to referenced primitive *To always know the size of your object, click the hammer icon to find what the size of editable poly/mesh is